staging: rtl8712: r8712_disconnectCtrlEx_cmd(): Change return type

Change return type of function r8712_disconnectCtrlEx_cmd from u8 to
void as its return value is never stored, checked or otherwise used.
Modify its return statements accordingly.

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