ath6kl: Add debugfs interface to dump diagnostic registers from firmware

To dump a particular register:

    echo <reg_addr> > <debugfs_root>/ieee80211/phyX/ath6kl/reg_addr

To dump the entire register set:

    echo 0 > <debugfs_root>/ieee80211/phyX/ath6kl/reg_addr

Register values will be available at:

    cat <debugfs_root>/ieee80211/phyX/ath6kl/reg_dump

kvalo: commit log cleanup, renamed few functions, removed a warning
message

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