MAINTAINERS: change IEEE 802.15.4 maintainer

This patch changes the IEEE 802.15.4 subsystem maintainer to Alexander Aring.
We discussed this change before via e-mail and I collected the acks from the
current maintainers.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Alexander Smirnov <alex.bluesman.sminov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>