staging: comedi: hwdrv_apci1564: remove useless code in apci1564_counter_insn_config()

The gate, trig, and ena, bits in the 'ctrl' are cleared at the start of
the function. There is no reason clear these bits for each step.

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>