Input: n64joy - fix return value check in n64joy_probe()

Input: n64joy - fix return value check in n64joy_probe()

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

Fixes: 3bdffa8ffb45 ("Input: Add N64 controller driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Lauri Kasanen <cand@gmx.com>
Link: https://lore.kernel.org/r/20210308122856.2177071-1-weiyongjun1@huawei.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>