mt76: fix return value check in mt76_wmac_probe()

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

Fixes: c8846e10 ("mt76: add driver for MT7603E and MT7628/7688")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>