e1000e: convert enums of register offsets and move #defines to regs.h

There are enough register offsets to warrant being in their own header
file, and doing so logically separates them from other header file content.
They have been converted from an enumerated data type to #defines as is
done in all the other Intel wired ethernet drivers.

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>