staging: comedi: adl_pci9118: introduce pci9118_ai_start_conv()

Writing any value to the "software trigger" register generates a trigger
pulse to start an A/D conversion. Introduce a helper function to clarify
this. For aesthetics, rename the define used for the register.

According to the datasheet the extra delay is not needed after starting
the conversion. Remove it.

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>