stating: gdm724x: remove explicit NULL comparison

This patch converts explicit NULL comparison to its shorter
equivalent form.
Done with coccinelle semantic patch:

@@
expression e;
@@

- e == NULL
+ !e

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>