staging: comedi: adv_pci1710: introduce pci171x_ai_read_sample()

Introduce a helper function to read an analog input sample, check for
channel dropout, and mask the sample to the maxdata of the subdevice.

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>