Bluetooth: l2cap: Don't double set the HCI_CONN_MGMT_CONNECTED bit

The bit is set and tested inside mgmt_device_connected(), therefore we
must not set it just outside the function.

Fixes: eeda1bf9

 ("Bluetooth: hci_event: Fix not indicating new connection for BIG Sync")
Signed-off-by: Archie Pusaka <apusaka@chromium.org>
Reviewed-by: Manish Mandlik <mmandlik@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>