ath10k: add back enum ath10k_bus

Commit 3a0861fffd223 ("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>
ath10k: refactor ath10k_init_download_firmware()

This is preparation for being able to download calibration data from a file.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
ath10k: retrieve calibration data from file

A frequent request have been to be able to provide calibration data from a
file as some of the AP devices store the calibration data on an MTD partition.
This patchset adds support for that and also makes it easier to add Device Tree
support later on.

The calibration data is found by using the id string provided by dev_name()
using this format:

cal-<bus>-<id>.bin

With PCI the id string contains bus, slot and func values. For example for a
PCI device in bus 2 slot 0, ath10k will try to retrieve a calibration data from
a file:

/lib/firmware/ath10k/cal-pci-0000:02:00.0.bin

The calibration data sequence is:

1. Check with request_firmware() if there's a calibration file
   ("cal-<bus>-<id>.bin") on the filesystem for this device. If yes, use that. If
   not, goto 2

2. Check if otp.bin is able to successfully load the calibration data
   from OTP. If yes, use that. If not, goto 3.

4. Print an error message that no calibration data found and stop driver
   initialization for this device.

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