bnxt_en: Fix incorrect multicast rx mask setting when not requested

We should setup multicast only when net_device flags explicitly
has IFF_MULTICAST set. Otherwise we will incorrectly turn it on
even when not asked.  Fix it by only passing the multicast table
to the firmware if IFF_MULTICAST is set.

Fixes: 7d2837dd

 ("bnxt_en: Setup multicast properly after resetting device.")
Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>