staging: comedi: amplc_pci224: only calc the pacer divisors once

When the cmd->scan_begin_src == TRIG_TIMER the divisors needed to
generate the pacer time are calculated in the (*do_cmdtest) to
validate the cmd->scan_begin_arg. The core always does the
(*do_cmdtest) before the (*do_cmd) so there is no reason to recalc
the divisors.

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