[BRIDGE]: Add support for NETIF_F_HW_CSUM devices

As it is the bridge will only ever declare NETIF_F_IP_CSUM even if all
its constituent devices support NETIF_F_HW_CSUM.  This patch fixes
this by supporting the first one out of NETIF_F_NO_CSUM,
NETIF_F_HW_CSUM, and NETIF_F_IP_CSUM that is supported by all
constituent devices.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>