bnxt_en: Support clearing of the IFF_BROADCAST flag.

Currently, the driver assumes IFF_BROADCAST is always set and always sets
the broadcast filter.  Modify the code to set or clear the broadcast
filter according to the IFF_BROADCAST flag.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>