ath6kl assumed cfg80211 passed to us power in dBm but it is in mBm.

Cc: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: NLuis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>