staging: rtl8188eu: core: Remove NULL test before vfree

vfree frees the virtually continuous memory area starting at addr.
If addr is NULL, no operation is performed. So NULL test is not needed
before vfree.

This was done using Coccinelle:

@@
expression x;
@@
-if (x != NULL)
    vfree(x);

@@
expression x;
@@

-if (x != NULL) {
vfree(x);
x = NULL;
-}

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