Bluetooth: RFCOMM: Refactor rfcomm_handle_pn()

Handle the scenarios like ignoring a response in non existing dlc,
correctly handle if received a command with invalid mtu etc.

Change-Id: Ib0bce9134bac3a0dead03798f859af54873a70c1
Signed-off-by: Jaganath Kanakkassery <jaganathx.kanakkassery@intel.com>