wil6210: refactor disconnect flow

Separate sending command to the fw from the event handling function to
simplify the disconnect flow and track the from_event flag correctly.

Signed-off-by: Ahmad Masri <amasri@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>