rgbled: fixed detection of device on PX4v1

There is a serial EEPROM on the PX4IOv1 board that answers on I2C
address 0x55. We need some extra I2C transfers to ensure we are
talking to a real RGBLED device.
rgbled: fixed getopt() handling

this allows the -a option to be used, for example
  rgbled -a 0x55 start
rgbled: don't try the same bus twice

on PX4v1 the external I2C bus is the same as the LED bus