fakelb: allow to run as monitor

For my RIOT-OS in userspace experiments I need to create a fakelb
monitor interface. The fakelb doesn't filter anything on L2 and is a
purely raw interface. Because nl802154 checks on promiscuous mode which
need to supported by creating monitors this patch adds some no-op
promiscuous mode setting and the promiscuous flag.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>