igb: add support for x2 link width configurations

When device is on PCIe link trained as x2 the driver is currently reporting
link width as "unknown".  The original patch provided by Myron adds the x2
link support and my changes are cosmetic to clean up the readability of the
conditional operators.

Based on work by: Myron Stowe <myron.stowe@hp.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>