Change reference cardinality from MULTIPLE to AT_LEAST_ONE so the serial port provider can be used immediately when ThingHandlers are initialized.
Fixes#937
This would always result in the following logging on startup:
[WARN ] [serial.internal.SerialPortManagerImpl] - No SerialPortProvider found for: /dev/ttyUSB0
Bindings without serial port reconnection logic would need to be manually restarted (Z-Wave Binding).
Signed-off-by: Wouter Born <github@maindrain.net>
