Bluetooth: rfcomm: Support dynamic channel allocation

In the function `bt_rfcomm_server_register()`, the channel cannot be
dynamic allocated. And only pre-set channel is supported.

Improve the function `bt_rfcomm_server_register()` to support the
dynamic channel allocation if the passed channel is zero.

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