Bluetooth: Fix sending HCI_Disconnect only when connected

HCI_Disconnect should only be sent after connection is established.
If connection is not yet established and HCI_Disconnect is called
then disconnection complete will be received with a handle which
does not exist and hence this event will be ignored.
But as mgmt.c will not receive this event, its variable for pending
command is not cleared.This will result in future Disconnect commands
for that BD Address to be blocked with error busy.

Signed-off-by: Vishal Agarwal <vishal.agarwal@stericsson.com>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>