e1000e: cleanup Rx checksum offload code

1) cleanup whitespace in e1000_rx_checksum() function header comment
2) do not check hardware checksum when Rx checksum is disabled
3) reduce duplicated calls to le16_to_cpu() by just using it within
   e1000_rx_checksum() instead of in each call to the function

v2: use swab16 instead of le16_to_cpu & htons and corrected type for the
passed-in csum

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>