igb: update feature flags supported in ethtool

This driver is currently using HW_CSUM which is not correct.  Update this
to use the IP_CSUM and IPV6_CSUM flags.  In addition consolidate the TSO
flag setting.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>