igbvf: cleanup flags and allow for rx checksum to be disabled

This patch cleans up a number of unused or unneeded feature flags.  As a
result of these changes the user should now be able to enable or disable rx
checksumming via ethtool.

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>