ethdev: change link status get functions to return int

ethdev: change link status get functions to return int

Change rte_eth_link_get() and rte_eth_link_get_nowait() return value
from void to int and return negative errno values in case of error
conditions.

Return value of link_update callback is ignored since the callback
returns not errors but whether link up status has changed or not.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>