Bluetooth: OBEX: fix incorrect error code in obex_transport_disconn

The function obex_transport_disconn() was always returning -EINVAL
regardless of the actual error code returned by the transport
disconnect operation.

Fix this by returning the actual error code from the transport
disconnect call instead of the hardcoded -EINVAL value.

Signed-off-by: Can Wang <can.wang@nxp.com>