staging: comedi: pcmmio: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change `interrupt_pcmmio()` and `ai_rinsn()` in the "pcmmio" module to
use unsigned sample values for consistency.

Also, make the order in which `interrupt_pcmmio()` writes the two sample
values (each actually containing up to 16 1-bit sample values)
independent of the host byte ordering.

Note that this module is a mess, so please excuse the checkpatch
warnings.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>