staging: rtl8712: r8712_setstakey_cmd(): Change return type

Change return type of function r8712_setstakey_cmd() to void as its
return value is never stored, checked, or otherwise used.

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