[PATCH] iwlwifi: add debugfs rate scale stats

This patch adds rates scale statistics to debugfs:
$ cat /sys/kernel/debug/ieee80211/phy<X>/stations/<mac>/rate_stats_table

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>