ethdev: allow stop function to return an error

ethdev: allow stop function to return an error

Change rte_eth_dev_stop() return value from void to int
and return negative errno values in case of error conditions.
Also update the usage of the function in ethdev according to
the new return type.

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>