ath10k: add log level configuration for fw_dbglog

Introduce an optional log level configuration for the existing debugfs fw_dbglog file. It
allows users to configure the desired log level for firmware dbglog messages.

To configure log level as WARN:

echo 0xffffffff  2 > /sys/kernel/debug/ieee80211/phy0/ath10k/fw_dbglog

The values are:

VERBOSE		0
INFO		1
WARN		2
ERR		3

Signed-off-by: SenthilKumar Jegadeesan <sjegadee@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>