ath5k: Use high bitrates for ACK/CTS

There was a confusion in the usage of the bits AR5K_STA_ID1_ACKCTS_6MB and
AR5K_STA_ID1_BASE_RATE_11B. If they are set (1), we will get lower bitrates for
ACK and CTS. Therefore ath5k_hw_set_ack_bitrate_high(ah, false) actually
resulted in high bitrates, which i think is what we want anyways. Cleared the
confusion and added some documentation.

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