staging: comedi: ni_labpc: introduce labpc_counter_set_mode()

Introduce labpc_counter_set_mode() which is a wrapper around the
i8254_set_mode() helpers to program the 8254 timers. Use the new
function instead of directly writing to the timer mode register
with a "magic" number.

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