ixgbe: Cleanup PCIe bus speed info

PCIe connections should be expressed as GT/s (GigaTransfers per second)
instead of the current Gb/s (Gigabits per second).  In addition, it is
incorrect because (due to PCIe gen 1 & 2 having a 20% overhead) the
actually data rate, when expressed in Gb/s, is only 80% of the rate of
GT/s.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Evan Swanson <evan.swanson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>