usb: gadget: udc: lpc32xx: fix return value check in lpc32xx_udc_probe()

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

This issue was detected by using the Coccinelle software.

Fixes: 408b56ca ("usb: gadget: udc: lpc32xx: simplify probe")
Acked-by: Sylvain Lemieux <slemieux.tyco@gmail.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>