ath9k: Combine legacy and 11n rc statistics

This patch combines the legacy and 11n rcstats into one, using the normal
rate table indices instead of two separate indices for each mode.  Legacy
rates also get all of the PER and retry information, now, too.

Signed-off-by: Jeff Hansen <x@jeffhansen.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>