mac802154: use header operations to create/parse headers

Use the operations on 802.15.4 header structs introduced in a previous
patch to create and parse all headers in the mac802154 stack. This patch
reduces code duplication between different parts of the mac802154 stack
that needed information from headers, and also fixes a few bugs that
seem to have gone unnoticed until now:

 * 802.15.4 dgram sockets would return a slightly incorrect value for
   the SIOCINQ ioctl
 * mac802154 would not drop frames with the "security enabled" bit set,
   even though it does not support security, in violation of the
   standard

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>