bluetooth: rfcomm: fix issue of sending buf invalid

There is a change (commit no.: 93d0eac8) that if
the `ref` of sending buf is not 1, the error code
`-EINVAL` will be returned from bt_conn_send_cb.

It causes the RFCOMM functionality cannot work
properly.

Remove the ref operation from the buf to be sent
to fix the issue.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>