Driver: Vmxnet3: set CHECKSUM_UNNECESSARY for IPv6 packets

For IPv6, if the device indicates that the checksum is correct, set
CHECKSUM_UNNECESSARY.

Reported-by: Subbarao Narahari <snarahari@vmware.com>
Signed-off-by: Shrikrishna Khare <skhare@vmware.com>
Signed-off-by: Jin Heo <heoj@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>