comedi vmk80xx: simplify rinsn output calculation

vmk80xx_di_rinsn() and vmk80xx_do_rinsn() extract the required channel
data by inconsistent and overly-complex algorithms. Simplify them both.

Signed-off-by: J. Ali Harlow <ali@avrc.city.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>