ath9k: Setup short preamble properly in rate registration

This patch fixes an issue where hw_value_short was not being filled
with the proper ratecode during rate registration, but was being used
in the RX path for decoding the packet rate.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>