ath10k: fetch firmware images in a loop

To make it easier to handle minimum and maximum firmware API numbers convert
the firmware fetch functionality to a loop. If no firmware image is found print
an error with minimum and maximum API numbers and the name of firmware
directory. This is needed when we switch to using request_firmware_direct()
which doesn't print any errors anymore.

Also add a new function for creating the fw file name dynamically which makes it
easier to add new bus support, for example SDIO and USB, later.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
[kvalo@qca.qualcomm.com: remove sdio/usb part, new error message, clarify commit log]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>