staging: comedi: serial2002: Fix unnecessary cast styling issue

checkpatch.pl warns that a cast of 1 to (long long) is unnecessary.
Remove the cast and also add code to check and make sure that
maxdata_list[chan] is not shifted too far, as suggested by Ian
Abbott.

Signed-off-by Chris Cesare <chris.cesare@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>