staging: rtl8712: r8712_enqueue_cmd_ex(): Change return type

Change return type of function r8712_enqueue_cmd_ex from u8 to void as
its return value is never stored, checked or otherwise used. Modify
return statements accordingly.

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