mac802154: Move an skb free within the rx path

It may appear clearer to free the skb at the end of the path rather than
in the middle, within a helper.

Move kfree_skb() from the end of __ieee802154_rx_handle_packet() to
right after it in the calling function ieee802154_rx(). Doing so implies
reworking a little bit the exit path.

Suggested-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20221026093502.602734-2-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>