ieee802154: add helpers for frame control checks

This patch introduce two static inline functions. The first to get the
frame control field from an sk_buff. The second is for checking on the
acknowledgment request bit on the frame control field. Later we can
introduce more functions to check on the frame control fields.

These will deprecate the current behaviour which requires a
host-byteorder conversion and manually bit handling.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>