staging: rtl8192ee: Pass large struct by const reference

struct rtl_stats is rather huge (152 bytes), and since
rtl92ee_rx_command_packet() does not modify it, it might as
well be passed by const reference.

Reported by Coverity: CID 1222131

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