usb: dwc3: Fix an IS_ERR() vs NULL check in dwc3_power_off_all_roothub_ports()

The ioremap() function doesn't return error pointers, it returns NULL on
error.  Update the check.

Fixes: 2d2a3349 ("usb: dwc3: Add workaround for host mode VBUS glitch when boot")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/71499112-4ed3-489a-9a56-b4a8ab89cd05@moroto.mountain


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>