staging: rtl8188eu: Standardize test for NULL.

The test for NULL of the return variable of functions was changed from
(ret == NULL) to !ret to match the standard.

Coccinelle was used with semantic patch:
@@
expression e;
identifier id, f;
statement S;
@@

f(...) { <+...

id =
\(kmalloc\|devm_kzalloc\|kmalloc_array\|devm_ioremap
\|usb_alloc_urb\|alloc_netdev\|dev_alloc_skb\) (...)

... when any
    when != id = e

+ if (!id)
- if (\(NULL == id\|id == NULL\))
S

...+> }

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/os_dep/usb_ops_linux.c

@@ -252,7 +252,7 @@ static int usbctrl_vendorreq(struct adapter *adapt, u8 request, u16 value, u16 i
 	/*  Acquire IO memory for vendorreq */
 	pIo_buf = kmalloc(MAX_USB_IO_CTL_SIZE, GFP_ATOMIC);
 
-	if (pIo_buf == NULL) {
+	if (!pIo_buf) {
 		DBG_88E("[%s] pIo_buf == NULL\n", __func__);
 		status = -ENOMEM;
 		goto release_mutex;