ixgbe: add cx4 device ID

This is a simple device ID add for adapters that support
CX4 (copper infiniband style cable) connectors for 10GbE.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>