staging: rtl8188eu: replace explicit NULL comparison

Replace explicit NULL comparison with ! operator to
simplify code.

Found with Coccinelle script:
@@
expression ptr;
position p;
statement s0;
@@

ptr@p =
\(kmalloc\|devm_kzalloc\|kmalloc_array\|devm_ioremap\|usb_alloc_urb\|
alloc_netdev\|dev_alloc_skb\)(...)
... when != ptr

if (
(
+ !
ptr
- == NULL
)
) s0

Signed-off-by: Gargi Sharma <gs051095@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/os_dep/xmit_linux.c

@@ -72,7 +72,7 @@ int rtw_os_xmit_resource_alloc(struct adapter *padapter, struct xmit_buf *pxmitb
 
 	for (i = 0; i < 8; i++) {
 		pxmitbuf->pxmit_urb[i] = usb_alloc_urb(0, GFP_KERNEL);
-		if (pxmitbuf->pxmit_urb[i] == NULL) {
+		if (!pxmitbuf->pxmit_urb[i]) {
 			DBG_88E("pxmitbuf->pxmit_urb[i]==NULL");
 			return _FAIL;
 		}