usb: gadget: xilinx: fix devm_ioremap_resource() check

devm_ioremap_resource() returns IOMEM_ERR_PTR() and it never
returns NULL, fix the check to prevent access to invalid
virtual address.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>