staging: vt6656: replace explicit NULL comparison with ! operator

Replace explicit NULL comparison with ! operator to simplify code.

Found with Coccinelle:
@@
expression e;
statement s0, s1;
@@

if (
(
+ !
e
- == NULL
 || ...
)
 ) s0 else s1

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>