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>

drivers/net/wireless/ath/main.c

@@ -90,3 +90,10 @@ void ath_printk(const char *level, const struct ath_common* common,
 	va_end(args);
 }
 EXPORT_SYMBOL(ath_printk);
+
+const char *ath_bus_type_strings[] = {
+	[ATH_PCI] = "pci",
+	[ATH_AHB] = "ahb",
+	[ATH_USB] = "usb",
+};
+EXPORT_SYMBOL(ath_bus_type_strings);