ath6kl: Add debugfs support to write a chip register

To write a value to register:
echo <register_offset>=<register_value> > <degfs_root>/ieee80211/phyX/ath6kl/reg_write

kvalo: rename file to reg_write to follow the style of other debugfs files

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