ieee802154: dgram: cleanup set of broadcast panid

This patch is only a cleanup to use the right define for a panid field.
The broadcast address and panid broadcast is still the same value.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Cc: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>