staging: comedi: adv_pci1710: always enable PCI171x_PARANOIDCHECK code

This define enables code that checks for analog input channel dropout
when reading samples. The define is enabled so we might as well always
enable the code and remove the define.

Factor out the common channel dropout detect code as a helper function
and cleanup 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>