staging: comedi: Remove unecessary function and it's call

Function aref_invalid has unreachable code as it right now
just returns 0 after declaring a variable.
This patch removes this function and it's single call.

As it is static so it's obvious that it is not used anywhere
other than this file or somewhere, where this is included.
I also build tested it.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>