usb: core: usbport: Use proper LED API to fix potential crash

Calling brightness_set manually isn't safe as some LED drivers don't
implement this callback. The best idea is to just use a proper helper
which will fallback to the brightness_set_blocking callback if needed.

This fixes:
[ 1461.761528] Unable to handle kernel NULL pointer dereference at virtual address 00000000
(...)
[ 1462.117049] Backtrace:
[ 1462.119521] [<bf228164>] (usbport_trig_port_store [ledtrig_usbport]) from [<c023f758>] (dev_attr_store+0x20/0x2c)
[ 1462.129826]  r7:dcabc7c0 r6:dee0ff80 r5:00000002 r4:bf228164
[ 1462.135511] [<c023f738>] (dev_attr_store) from [<c0169310>] (sysfs_kf_write+0x48/0x4c)
[ 1462.143459]  r5:00000002 r4:c023f738
[ 1462.147049] [<c01692c8>] (sysfs_kf_write) from [<c0168ab8>] (kernfs_fop_write+0xf8/0x1f8)
[ 1462.155258]  r5:00000002 r4:df4a1000
[ 1462.158850] [<c01689c0>] (kernfs_fop_write) from [<c0100c78>] (__vfs_write+0x34/0x120)
[ 1462.166800]  r10:00000000 r9:dee0e000 r8:c000fc24 r7:00000002 r6:dee0ff80 r5:c01689c0
[ 1462.174660]  r4:df727a80
[ 1462.177204] [<c0100c44>] (__vfs_write) from [<c0101ae4>] (vfs_write+0xac/0x170)
[ 1462.184543]  r9:dee0e000 r8:c000fc24 r7:dee0ff80 r6:b6f092d0 r5:df727a80 r4:00000002
[ 1462.192319] [<c0101a38>] (vfs_write) from [<c01028dc>] (SyS_write+0x4c/0xa8)
[ 1462.199396]  r9:dee0e000 r8:c000fc24 r7:00000002 r6:b6f092d0 r5:df727a80 r4:df727a80
[ 1462.207174] [<c0102890>] (SyS_write) from [<c000fa60>] (ret_fast_syscall+0x0/0x3c)
[ 1462.214774]  r7:00000004 r6:ffffffff r5:00000000 r4:00000000
[ 1462.220456] Code: bad PC value
[ 1462.223560] ---[ end trace 676638a3a12c7a56 ]---

Reported-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Fixes: 0f247626cbb ("usb: core: Introduce a USB port LED trigger")
Cc: stable@vger.kernel.org # 4.9+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/ledtrig-usbport.c

@@ -74,8 +74,7 @@ static void usbport_trig_update_count(struct usbport_trig_data *usbport_data)
 
 	usbport_data->count = 0;
 	usb_for_each_dev(usbport_data, usbport_trig_usb_dev_check);
-	led_cdev->brightness_set(led_cdev,
-				 usbport_data->count ? LED_FULL : LED_OFF);
+	led_set_brightness(led_cdev, usbport_data->count ? LED_FULL : LED_OFF);
 }
 
 /***************************************
@@ -228,12 +227,12 @@ static int usbport_trig_notify(struct notifier_block *nb, unsigned long action,
 	case USB_DEVICE_ADD:
 		usbport_trig_add_usb_dev_ports(usb_dev, usbport_data);
 		if (observed && usbport_data->count++ == 0)
-			led_cdev->brightness_set(led_cdev, LED_FULL);
+			led_set_brightness(led_cdev, LED_FULL);
 		return NOTIFY_OK;
 	case USB_DEVICE_REMOVE:
 		usbport_trig_remove_usb_dev_ports(usbport_data, usb_dev);
 		if (observed && --usbport_data->count == 0)
-			led_cdev->brightness_set(led_cdev, LED_OFF);
+			led_set_brightness(led_cdev, LED_OFF);
 		return NOTIFY_OK;
 	}