n64cart: fix return value check in n64cart_probe()

n64cart: fix return value check in n64cart_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: 0e188e085791 ("block: Add n64 cart driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20210909090608.2989716-1-yangyingliang@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>