aesni_mb: fix wrong return value

aesni_mb: fix wrong return value

cryptodev_aesni_mb_init was returning the device id of
the device just created, but rte_eal_vdev_init
(the function that calls the first one), was expecting 0 or
negative value.
This made impossible to create more than one aesni_mb device
from command line.

Fixes: 924e84f87306 ("aesni_mb: add driver for multi buffer based crypto")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>