ath5k: simplify MIB counters

Let's keep MIB counter statistics in our own statistics structure and only
convert it to ieee80211_low_level_stats when needed by mac80211. Also we don't
need to read profile count registers in the MIB interrupt (they don't trigger
MIB interrupts).

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>