usb: dwc3: pci: Fix return value check in dwc3_byt_enable_ulpi_refclock()

In case of error, the function pcim_iomap() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: 7740d04d901d ("usb: dwc3: pci: Enable ULPI Refclk on platforms where the firmware does not")
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/dwc3-pci.c

@@ -85,8 +85,8 @@ static int dwc3_byt_enable_ulpi_refclock(struct pci_dev *pci)
 	u32		value;
 
 	reg = pcim_iomap(pci, GP_RWBAR, 0);
-	if (IS_ERR(reg))
-		return PTR_ERR(reg);
+	if (!reg)
+		return -ENOMEM;
 
 	value = readl(reg + GP_RWREG1);
 	if (!(value & GP_RWREG1_ULPI_REFCLK_DISABLE))