net: dsa: rzn1-a5psw: fix a NULL vs IS_ERR() check in a5psw_probe()

The devm_platform_ioremap_resource() function never returns NULL.
It returns error pointers.

Signed-off-by: Peng Wu <wupeng58@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Clément Léger <clement.leger@bootlin.com>
Link: https://lore.kernel.org/r/20220628130920.49493-1-wupeng58@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>