Mixing UART C3 registers with C2 bit flags

[ssilverman]

https://github.com/chrisstaite/TeensyDmx/blob/da868f95e127f69c77cb30067929e074180d643f/TeensyDmx.cpp#L2286

Seems to be mixing UART register C3 with register C2 bit flags; also for UARTs 1-5.