usb: typec: fusb302: Add __printf attribute to fusb302_log function

Add __printf attribute to fusb302_log function, so that we get
compiler warnings when specifying wrong vararg parameters.

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