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: 7740d04d ("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>