ath10k: add back enum ath10k_bus

Commit 3a0861ff ("ath10k: remove ath10k_bus") removed enum ath10k_bus
because it was not used for anything at the time. But now it's needed for for
retrieving the right calibration data file so add it back. Only new addition is
ath10k_bus_str().

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>