ixgbe: disable RSC when Rx checksum is off

Disabling Rx checksumming leads to performance degradation due to
RSC causing packets to have incorrect checksums.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Evan Swanson <evan.swanson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>