staging: rtl8712: _r8712_free_sta_priv(): Change return type

Change return type of function _r8712_free_sta_priv from u32 to void as
it always returns _SUCCESS and this return value is never used.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>