Staging: rtl8712: remove void function return; statement

This patch fixes checkpatch.pl warning in files of rtl8712
WARNING : void function return statement are not generally useful

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>