Clean up LED implementation

[ggVGc]

The FLEXIO LED-driver could use some improvements and cleanup, especially in the DMA deparement (such as making the DMA channel configurable).

Not urgent, but should be done when time allows.

[DavidMenting]

Is there anything that needs to be cleaned up apart form having the DMA channel configurable? Seeing that we're targeting one application I don't think having configurable DMA is as important as it would be in - for instance - a Teensy or other development board.