ath10k: silence firmware file probing warnings

Firmware files are versioned to prevent older
driver instances to load unsupported firmware
blobs. This is reflected with a fallback logic
which attempts to load several firmware files.

This however produced a lot of unnecessary
warnings sometimes confusing users and leading
them to rename firmware files making things even
more confusing.

Hence use request_firmware_direct() which does not
produce extra warnings. This shouldn't really
break anything because most modern systems don't
rely on udev/hotplug helpers to load firmware
files anymore. For example it was confirmed that
LEDE does not user helper.

This also fixes a 60 second delay per _each_
unexistent firmware/calibration file with distros
which have CONFIG_FW_LOADER_USER_HELPER_FALLBACK
enabled, RHEL being a notable example. Using
ath10k with firmware-2.bin this might end up
into a five minute delay in boot.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
[kvalo@qca.qualcomm.com: add more info to the commit log]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>