drivers: staging: rtl8712: Change form of NULL comparisons

Change null comparisons of the form x == NULL to !x.
This was done using Coccinelle.

@@
expression e;
@@
- e == NULL
+ !e

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>