ps2: end data byte at rising clock edge of the stop bit

ps2: end data byte at rising clock edge of the stop bit

Data byte transmission is complete when the STOP bit was communicated.
End the STOP bit at the rising CLOCK edge of the 11th bit time, do not
rely on the host's clock inhibit after data transmission. This avoids
the unexpected expansion of a STOP bit well into the next data byte.

This fixes bug #1460, where absence of host activity after the first
data byte loses synchronization to the input stream.

Rephrase pin level sampling while we are here. Reflect that the clock
line is used for .wait() conditions, but its level is not evaluated.
Only data line levels get processed.

Comment on the implementation's assumption of specific input data (the
device as transmitter, host transmit may not be supported, or can result
in unexpected output). Which may be acceptable yet maintainers need to
remain aware.