staging: comedi: s626: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Use unsigned types consistently in the "s626" module when dealing with
sample values.

Rewrite `s626_reg_to_uint()` as it can be done with a one-liner.

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