staging: rtl8723au: core: Remove uneeded return variable

This patch removes uneeded return variables, using only
'return _SUCCESS' instead.
It fixes the following warning detected by coccinelle:
Unneeded variable.

It was done using the following semantic patch:

@@
identifier ret;
type T;
expression e;
@@

-T ret = e;
 ... when != ret
     when strict
-return ret;
+return e;

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>