mac80211: minstrel, fix memory corruption

minstrel doesn't count max rate count in fact, since it doesn't use
a loop variable `i' and hence allocs space only for bitrates found in
the first band.

Fix it by involving the `i' as an index so that it traverses all the
bands now and finds the real max bitrate count.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>