staging: comedi: pcmmio: cleanup ai_rinsn()

Rename this function to have namespace associated with the driver.

Refactor the function to remove the extra write to the command register
between each ADC conversion. We only need to do one dummy conversion in
order to flush the serial ADC. After that each command will return the
result of the previous conversion.

Define the register map for the analog input registers.

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>