igb: Read register for latch_on without return value

This patch changes register read to "just-read" without returning a value
for hardware to accurately latch the register value.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>