USB: serial: io_ti: fix I/O after disconnect

Cancel the heartbeat work on driver unbind in order to avoid I/O after
disconnect in case the port is held open.

Note that the cancel in release() is still needed to stop the heartbeat
after late probe errors.

Fixes: 26c78daa ("USB: io_ti: Add heartbeat to keep idle EP/416
ports from disconnecting")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>