ethdev: format link status text

ethdev: format link status text

There is new link_speed value introduced. It's INT_MAX value which
means that speed is unknown. To simplify processing of the value
in application, new function is added which convert link_speed to
string. Also dpdk examples have many duplicated code which format
entire link status structure to text.

This commit adds two functions:
  * rte_eth_link_speed_to_str - format link_speed to string
  * rte_eth_link_to_str - convert link status structure to string

Signed-off-by: Ivan Dyukov <i.dyukov@samsung.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>