staging: rtl8192u: ieee80211: Use !x in place of NULL comparison

Change NULL comparison to Boolean negation.Issue found using Coccinelle

Semantic patch used to solve the problem is as follows:

@replace_rule@
expression e;
@@

- e == NULL
+ !e

Signed-off-by: Vatsala Narang <vatsalanarang@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>