Staging: comedi: unlock on error in usbdux_ao_inttrig()

If we had an invalid trignum (anything other than zero is invalid) then
we returned without unlocking.  I've modified this function to just have
one return point.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>