ixgbevf: Make use of NETIF_F_RXCSUM instead of keeping our own flag

The IXGBE_FLAG_RX_CSUM_ENABLED flag is redundant since NETIF_F_RXCSUM is
keeping the value we want to already have.  As such we can drop the
redundant flag and just make use of NETIF_F_RXCSUM.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>