igb: fix assignment on big endian machines

The igb driver was trying hard to be sparse correct, but somehow
ended up converting a variable into little endian order and then
tries to OR something with it.

A much plainer way of doing things is to leave all variables and
OR operations in CPU (non-endian) mode, and then convert to
little endian only once, which is what this change does.

This probably fixes a bug that might have been seen only on
big endian systems.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>