mt7601u: null check the allocation

devm_kzalloc may fail and return NULL. So the null check is needed.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>