phy: bcm-ns-usb3: fix MDIO_BUS dependency

The driver attempts to 'select MDIO_DEVICE', but the code
is actually a loadable module when PHYLIB=m:

drivers/phy/broadcom/phy-bcm-ns-usb3.o: In function `bcm_ns_usb3_mdiodev_phy_write':
phy-bcm-ns-usb3.c:(.text.bcm_ns_usb3_mdiodev_phy_write+0x28): undefined reference to `mdiobus_write'
drivers/phy/broadcom/phy-bcm-ns-usb3.o: In function `bcm_ns_usb3_module_exit':
phy-bcm-ns-usb3.c:(.exit.text+0x18): undefined reference to `mdio_driver_unregister'
drivers/phy/broadcom/phy-bcm-ns-usb3.o: In function `bcm_ns_usb3_module_init':
phy-bcm-ns-usb3.c:(.init.text+0x18): undefined reference to `mdio_driver_register'
phy-bcm-ns-usb3.c:(.init.text+0x38): undefined reference to `mdio_driver_unregister'

Using 'depends on MDIO_BUS' instead will avoid the link error.

Fixes: af850e14a7ae ("phy: bcm-ns-usb3: add MDIO driver using proper bus layer")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/phy/broadcom/Kconfig

@@ -30,8 +30,8 @@ config PHY_BCM_NS_USB3
 	tristate "Broadcom Northstar USB 3.0 PHY Driver"
 	depends on ARCH_BCM_IPROC || COMPILE_TEST
 	depends on HAS_IOMEM && OF
+	depends on MDIO_BUS
 	select GENERIC_PHY
-	select MDIO_DEVICE
 	help
 	  Enable this to support Broadcom USB 3.0 PHY connected to the USB
 	  controller on Northstar family.