mt76: usb: remove skb check in mt76x{0,2}u mcu routines

Remove mt76_mcu_msg_alloc return value check since it is already
evaluated in __mt76x02u_mcu_send_msg

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>