af_ieee802154: fix check on broadcast address

This patch fixes an issue which was introduced by commit
b70ab2e8

 ("ieee802154: enforce
consistent endianness in the 802.15.4 stack").

The correct behaviour should be a check on the broadcast address field
which is 0xffff.

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