ixgbe: fix bcast packets Rx on VF after promisc removal

After a VF requested to remove the promiscuous flag on an interface, the
broadcast packets are not received anymore. This breaks some protocols
like ARP.

In ixgbe_update_vf_xcast_mode(), we should keep the IXGBE_VMOLR_BAM
bit (Broadcast Accept) on promiscuous removal.

This flag is already set by default in ixgbe_set_vmolr() on VF reset.

Fixes: 8443c1a4

 ("ixgbe, ixgbevf: Add new mbox API xcast mode")
Cc: stable@vger.kernel.org
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>