staging: rtl8712: r8712_addbareq_cmd(): Change return type

Change return type of the function r8712_addbareq_cmd from u8 to void as
its return value is not stored, checked or otherwise used. Also modify
its return statements accordingly.

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