ethdev: fix link speed helper documentation

ethdev: fix link speed helper documentation

When generating the documentation, a new warning can be seen:

.../dpdk/lib/librte_ethdev/rte_ethdev.h:2441:
  warning: argument 'link_speed' of command @param is not found in the
  argument list of rte_eth_link_speed_to_str(uint32_t speed_link)
.../dpdk/lib/librte_ethdev/rte_ethdev.h:2455: warning: The following
  parameters of rte_eth_link_speed_to_str(uint32_t speed_link) are not
  documented: parameter 'speed_link'

Align the function prototype to its doxygen description.

Fixes: fbf931c9c392 ("ethdev: format link status text")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>