staging: rtl8821ae: Pass large struct by const reference

struct rtl_stats is rather huge (152 bytes), and since
rtl8812ae_rx_command_packet_handler() does not modify it, it might as
well be passed by const reference.

Reported by Coverity: CID 1167285

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>