Staging: comedi: drivers: comedi_test: Avoid multiple line dereference

Fix checkpatch warning "Avoid multiple line dereference"
using a pointer variable to avoid line wrap.

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>