staging: comedi: das16m1: don't calc pacer divisors twice

The analog input async command can use the pacer for the convert_src. The
(*do_cmdtest) 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.

Refactor das16m1_set_pacer() 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.

Tidy up das16m1_cmd_exec() a bit. The pacer only needs to be set when the
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>