ath10k: add debugfs file to control firmware dbglog

Firmware dbglogs can be now enabled through fw_dbglog file. To enable all
possible log messages run:

echo 0xffffffff > /sys/kernel/debug/ieee80211/phy0/ath10k/fw_dbglog

And to put back firmare defaults use 0x0:

echo 0x0 > /sys/kernel/debug/ieee80211/phy0/ath10k/fw_dbglog

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
(cherry picked from commit f118a3e5)