staging: comedi: cb_pcidas: don't calc ai pacer divisors twice

The analog input async command can use the pacer for the scan_bagin_src
or the convert_src. The (*do_cmdtest) validates that only one of these
sources has the TRIG_TIMER selected and calculates the divisors when
validating the cmd argument.

There is no reason to recalc the divisors in the (*do_cmd). Just use
the values from the private data.

Also, refactor cb_pcidas_load_counters() to use the i8254_set_mode()
and i8254_write() helpers instead of i8254_load(). This allows us to
use the I8254_* defines when setting the mode to clarify the code.

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>