e1000e: fix obscure comments

The interface to the device flash was modified in i219 and later HW.
This patch better describes the change and the impact on the driver.

CC: John W Linville <linville@tuxdriver.com>
Reported-by: John W Linville <linville@tuxdriver.com>
Signed-off-by: Yanir Lubetkin <yanirx.lubetkin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>