e1000e: SerDes autoneg flow control

Enables flow control to be set in SerDes autoneg mode. This is what is
done for copper, but relies on a different set of register/bit checks
since this is all done within the Mac registers.

Remove inapplicable comment in defines.h

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>