mac802154: add synchronization handling

This patch adds synchronization handling in start and stop driver ops
calls. This patch is mostly grab from mac80211 which was introduced by
commit ea77f12f ("mac80211: remove
tasklet enable/disable"). This is to be sure that we don't run into same
issues.

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