usb: typec: fusb302: Check vconn is off when we start toggling

The datasheet says the vconn MUST be off when we start toggling. The
tcpm.c state-machine is responsible to make sure vconn is off, but lets
add a WARN to catch any cases where vconn is not off for some reason.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>