igb: Changed LEDs blink mechanism to include designs using cathode

This patch addresses the changes needed to make LEDs work properly with
negative logic. This implementation uses LED Invert bit to reverse the
logic issue that occurred when LEDs are driven by cathode. Keep LEDs
blinking for SerDes devices. Also made changes to magic number and the
for loop to reduce number of shifts.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>