VWI: Reduce default bit-rate

[mikaelpatel]

The current default bit-rate for VWI is 4000 bps. This results in a 8X higher timer interrupt frequency (32 k interrupts per second). This load may be to high for ATtiny and applications with both VWI RX and TX.

There are two alternatives; 1) reduce the default bit-rate to for instance 2000 bps, 2) reduce the number of samples per bit. The latest will require some adjustment of the VWI PLL.