igb: Enable reading of wake up packet

Currently, in igb_resume(), igb driver ignores the Wake Up Status (WUS)
and Wake Up Packet Memory (WUPM) registers. This patch enables the igb
driver to read the WUPM if the controller was woken by a wake up packet
that is not more than 128 bytes long (maximum WUPM size), then pass it
up the kernel network stack.

Signed-off-by: Kim Tatt Chuah <kim.tatt.chuah@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>