ath9k_htc: Rename firmware

Since the new FW requires backward incompatible host driver changes,
rename the FW to allow older driver versions to work with the
older FW.

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