mt76: remove mcu_msg_alloc

We almost always use patter like this:

       skb = mt76_mcu_msg_alloc(dev, &msg, sizeof(msg));
       return mt76_mcu_send_msg(dev, skb, CMD_FUN_SET_OP, wait_resp);

This is not needed, we can allocate skb in mcu_send_msg routine.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>