staging: wlan-ng: simplify NULL tests

Replace direct comparisons to NULL i.e. 'x == NULL' with '!x' for
consistency. Coccinelle semantic patch used:

@@
identifier func;
expression x;
statement Z;
@@

x = func(...);

if (
(
+	!
	x
-	== NULL
|
+	!
-	NULL ==
	x
)
   ) Z

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>