e1000e: iAMT connections drop on driver unload when jumbo frames enabled

The jumbo frame configuration in the MAC/PHY should be reverted on 82579
and newer parts when the interface is brought down (not just when the MTU
is changed back to standard frame size) otherwise iAMT connections (e.g.
SoL, IDE-R) will be dropped and cannot be re-acquired until the MTU is
changed again.

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>