mac802154: monitor: merge into iface implementation

This patch removes the monitor implementation file and put all monitor
stuff into iface file. It's now small enough to put all necessary
handling into iface.

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

diff --cc net/mac802154/Makefile
Simple merge

diff --cc net/mac802154/iface.c
Simple merge

diff --cc net/mac802154/monitor.c
Simple merge