mac802154: tx: fix synced xmit deadlock

This patch reverts 6001d522 ("mac802154: tx: don't allow if down while
sync tx"). This has side effects with stop callback which flush the
transmit workqueue. The stop callback will wait until the workqueue is
flushed and holding the rtnl lock. That means it can happen that the stop
callback waits forever because it try to lock the rtnl mutex which is
already hold by stop callback.

Cc: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>