staging: comedi: ni_at_ao: fix 8254 timer programming

staging: comedi: ni_at_ao: fix 8254 timer programming

As pointed out by Ian Abbott, the i8254_load() function does not
use the I8254_MODE* values to specify the "mode".

Refactor the 8254 timer programming to use the i8254_set_mode()
and i8254_write() helpers instead of i8254_load(). This allows
us to use the I8254_MODE* defines to clarify the code.

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>