mac802154: cleanup in rx path

This patch replace the sizeof(struct rx_work) with sizeof(*work)
and directly passing the skb in mac802154_subif_rx()

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