mac80211: Fix output of minstrels rc_stats

An integer overflow in the minstrel debug code prevented the
throughput to be displayed correctly. This patch fixes that,
by permutating operations like proposed by Pavel Roskin.

Signed-off-by: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>