ath9k: add a helper to get the string representation of ath_bus_type

This can be used when the ath_bus_type has to be presented in a log
message or firmware filename.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>