mt76: mt7915: check for devm_pinctrl_get() failure

If devm_pinctrl_get() fails then it leads to an error pointer
dereference.  Add a check to prevent that.

Fixes: 99ad32a4

 ("mt76: mt7915: add support for MT7986")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>