staging: comedi: das6402: convert driver to use the comedi_8254 module

This driver uses an 8254 timer to generate the pacer clock used for analog
input data conversion. Convert it to use the comedi_8254 module to provide
support for the 8254 timer.

Remove the unnecessary programming of timer 0. The private data 'count'
value is never set by the driver and the timer is reset to MODE0 when
the timers are initialized during the attach of the driver.

Remove the unnecessart convert_src check in the (*do_cmdtest). The only
valid convert_src is TRIG_TIMER.

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>