staging: greybus: camera: simplify NULL test

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>

drivers/staging/greybus/camera.c

@@ -1121,7 +1121,7 @@ static ssize_t gb_camera_debugfs_write(struct file *file,
 	       return -EINVAL;
 
 	kbuf = kmalloc(len + 1, GFP_KERNEL);
-	if (kbuf == NULL)
+	if (!kbuf)
 		return -ENOMEM;
 
 	if (copy_from_user(kbuf, buf, len)) {