ieee802154: fix skb get fc on big endian

This patch fixes ieee802154_get_fc_from_skb function on big endian
machines. The function get_unaligned_le16 converts the byte order to
host byte order but we want to keep the byte order like in mac header.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>