Bluetooth: Better locking when tearing down an HCI connection.

Restarting Riva will abruptly disconnect the hci device, which can
tear down the hci_conn struct on one processor while RFCOMM code
is running on another. This would keep l2cap_chan_del from being
executed while RFCOMM is in the middle of using a socket.

CRs-fixed: 351273
Change-Id: I37c82cac022a1d1c0fd60791f7c15b56e2415850
Signed-off-by: Ankit Verma <ankitv@codeaurora.org>