net: mediatek: fix checking for NULL instead of IS_ERR() in .probe

devm_ioremap_resource() returns ERR_PTR() value on error, it never
returns NULL, fix it and propagate the returned error upwards.

Fixes: 656e7052

 ("net-next: mediatek: add support for MT7623 ethernet")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>