staging: speakup: (coding style) Rewrite comparisons to NULL

Rewrite comparisons to NULL so they dont show any checkpatch errors anymore.
"vc_cons[i].d != NULL" => "vc_cons[i].d"

Signed-off-by: Christian Colic <colic.christian@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>