Fix possible NULL pointer dereference in mt76u_mcu_deinit routine that
can occur if initialization path fails before calling mt76u_mcu_init_rx

Fixes: ee676cd5 ("mt76: add driver code for MT76x2u based devices")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>