vxge: Fix multicast issues

Fix error in multicast flag check, add calls to restore the status of
multicast and promiscuous mode settings after change_mtu, and style
cleanups to shorten the function calls by using a temporary variable.

Signed-off-by: Jon Mason <jon.mason@exar.com>
Signed-off-by: Sreenivasa Honnur <sreenivasa.honnur@exar.com>
Signed-off-by: Ramkrishna Vepa <ramkrishna.vepa@exar.com>
Signed-off-by: David S. Miller <davem@davemloft.net>