ieee820154: add pan_id setting support

This patch adds support for setting pan_id via nl802154 framework.
Adding a comment because setting 0xffff as pan_id seems to be valid
setting. The pan_id 0xffff as source pan is invalid. I am not sure now
about this setting but for the current netlink interface this is an
invalid setting, so we do the same now. Maybe we need to change that
when we have coordinator support and association support.

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