staging: comedi: Improved readability of function comedi_nsamples_left.

Improve readability of comedi_nsamples_left:
a) Reduce nesting by using more return statements.
b) Declare variables scans_left and samples_left at start of function.
c) Change type of scans_Left to unsigned long long to avoid cast.

Signed-off-by: Chris Opperman <eklikeroomys@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>