e1000e: fix LED blink logic for designs with LEDs driven by cathode

When the LEDs are driven by cathode, the bit logic is reversed.  Use the
LED Invert bit to invert the logic.  Cleanup use of a magic number and
change the for loop increment to reduce the number of shifts.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>