Revise default bit-rates and bit-timing

[Nakakiyo092]

The default settings of the Sn and Yn commands are
provided only for sanity-check communication
between same-firmware devices on short wiring; they are not tuned for
arbitrary real-world bus environments. Configure the bit timing
explicitly via the s and y commands for production use.

The current defaults are too demanding for devices with imprecise
clocks: the gap between the nominal and data bit rate is wide,
and the sampling points themselves are set in a way that is
unforgiving to clock inaccuracy.

[Nakakiyo092]

sampling point 80% / 50%

#78 (reply in thread)