ath6kl: Set the sme_state to disconnected in disconnect() callback

After a successful completion of disconnect() driver needs to set
it's sme_state to SME_DISCONNECTED to be in sync with cfg80211
state.

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