ieee820154: add short_addr setting support

This patch adds support for setting short address via nl802154 framework.
Also added a comment because a 0xffff seems to be valid address that we
don't have a short address. This is a valid setting but we need
more checks in upper layers to don't allow this address as source address.
Also the current netlink interface doesn't allow to set the short_addr
to 0xffff. Same for the 0xfffe short address which describes a not
allocated short address.

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