MAINTAINERS: fix drivers/ieee802154/

This directory was moved to drivers/net/ieee802154/ by commit
31d178bf ("drivers/ieee802154: move ieee802154 drivers to net
folder").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>