mac802154: tx: add comment at sync xmit callback

This patch adds a warning that xmit_sync callback is deprecated and
should be removed soon. The 802.15.4 subsystem will not accept synced
drivers anymore.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>