vdpa/mlx5: Fix possible uninitialized return value

Initialize err local variable to return -EAGAIN if the asid cannot be
found thus avoiding returning uninitialized value.

Fixes: 8fcd20c3

 ("vdpa/mlx5: Support different address spaces for control and data")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20220811134010.952291-1-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>