staging: rtl8192u: 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/rtl8192u/ieee80211/ieee80211_rx.c

@@ -129,7 +129,7 @@ ieee80211_frag_cache_get(struct ieee80211_device *ieee,
 				    8 /* WEP */ +
 				    ETH_ALEN /* WDS */ +
 				    (IEEE80211_QOS_HAS_SEQ(fc)?2:0) /* QOS Control */);
-		if (skb == NULL)
+		if (!skb)
 			return NULL;
 
 		entry = &ieee->frag_cache[tid][ieee->frag_next_idx[tid]];