eal: fix check when retrieving current CPU affinity

eal: fix check when retrieving current CPU affinity

pthread_getaffinity_np returns a >0 value when failing.

This is mainly for the sake of correctness.
The only case where it could fail is when passing an incorrect cpuset
size wrt to the kernel.

Fixes: 2eba8d21f3c9 ("eal: restrict cores auto detection")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Rami Rosen <ramirose@gmail.com>