ieee802154: header_ops: fix frame control setting

Sometimes upper-layer protocols wants to generate a new mac header by
filling "struct ieee802154_hdr" only. These upper-layers sets for the
address settings the source and dest fields, but not the fc fields for
indicate the source and dest address mode. This patch changes the
"ieee802154_hdr_push" function so the fc address fields are set
according the source and dest fields of "struct ieee802154_hdr".

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