staging: comedi: das800: tidy up das800_ai_insn_read()

Introduce a couple helper functions to read the 16-bit analog input
sample and to wait for the analog conversion to complete.

Tidy up the das800_ai_insn_read() function by using the new helpers.
Also, remove the need for 'thisboard' in the function by checking the
subdevice 'maxdata' variable to determine the analog input resolution.

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>