wil6210: wait for disconnect completion

cfg80211_ops.disconnect() should wait for disconnect flow to
complete. If it does not, internal state becomes out of sync with
one in cfg80211. If one does stress test connect/disconnect
sequence, cfg80211 will issue next connect before disconnect
completed internally.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>