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>