staging: ste_rmi4: 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>

drivers/staging/ste_rmi4/synaptics_i2c_rmi4.c

@@ -912,7 +912,7 @@ static int synaptics_rmi4_probe
 		return -ENOMEM;
 
 	rmi4_data->input_dev = input_allocate_device();
-	if (rmi4_data->input_dev == NULL) {
+	if (!rmi4_data->input_dev) {
 		retval = -ENOMEM;
 		goto err_input;
 	}