ixgb: make sure jumbos stay enabled after reset

Currently a device reset (ethtool -r ethX) would cause the
adapter to fall back to regular MTU sizes.

Signed-off-by: Matheos Worku <matheos.worku@sun.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>