staging: comedi: ni_tiocmd: tidy up ni_tio_input_inttrig()

Remove the BUG_ON(counter == NULL). If this can never happen and it
if did the driver should have died long before this function is called.

For aesthetics, rename the local variable 'retval' to simply 'ret'.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>