netdev-dpdk: Add link speed to get_status().

netdev-dpdk: Add link speed to get_status().

Report the link speed of the device in netdev_dpdk_get_status()
function.

Link speed is already reported as part of the netdev_get_features()
function. However only link speeds defined in the OpenFlow specs are
supported so speeds such as 25 Gbps etc. are not shown. The link
speed for the device is available in Mbps in rte_eth_link.
This commit converts the link speed for a given dpdk device to an
easy to read string and reports it in get_status().

Suggested-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>