Disconnect before trying to switch to a different network

Disconnect before trying to switch to a different network

Previously, when wpa_supplicant received bgscan results with a preferred
network, it connected to that network without disconnecting from the
previous one. This might result in an inconsistent state of upper
layers.

Fix this by disconnecting from the current AP before connecting to the
new one when the network profile changes and there is an existing
connection.

Signed-off-by: Ayala Beker <ayala.beker@intel.com>