staging: comedi: das1800: don't calc pacer divisors twice

The analog input async command can use the pacer for the scan_begin_src
or 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.

For aesthetics, rename the setup_counters() function so it has namespace
associated with the driver. Refactor the function to use the values from
the private data and absorb das1800_set_frequency() to clarify the code.
Refactor the function 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.

This function will not fail so change the return type to void to simplify
the (*do_cmd) a bit.

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>