igb: clean up code for setting MAC address

Drop a bunch of hand written byte swapping code in favor of just doing the
byte swapping ourselves.  The registers are little endian registers storing
a big endian value so if we read the MAC address array as little endian
then we will get the CPU registers into the proper layout.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>