staging: rtl8712: r8712_setopmode_cmd(): Change return type

Change return type of function r8712_setopmode_cmd() from u8 to void as
its return value is never stored or checked or otherwise used.

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