reset: mchp: sparx5: fix return value check in mchp_sparx5_map_io()

reset: mchp: sparx5: fix return value check in mchp_sparx5_map_io()

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

Fixes: 453ed4283beb ("reset: mchp: sparx5: add switch reset driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210519141638.3052456-1-weiyongjun1@huawei.com
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>