net/mlx5: fix kvfree of uninitialized pointer spec

Currently when a call to  esw_vport_create_legacy_ingress_acl_group
fails the error exit path to label 'out' will cause a kvfree on the
uninitialized pointer spec.  Fix this by ensuring pointer spec is
initialized to NULL to avoid this issue.

Addresses-Coverity: ("Uninitialized pointer read")
Fixes: 10652f39 ("net/mlx5: Refactor ingress acl configuration")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>