i2c: Fix corner-case that can occur with LA triggers.

i2c: Fix corner-case that can occur with LA triggers.

Until now the I2C PD was basically ignoring the very first sample, and
using that as the initial 'oldscl'/'oldsda' value.

However, if your logic analyzers trigger on, say, SDA=low that will
result in a file where the first sample is really important since it
is the one which the PD will need to know that there's a falling edge
on SDA.

Thus, assume both SCL and SDA are high/1 when the PD starts. This is
a good assumption since both pins have pullups on them in practice
and are thus high/1 when the bus is idle.

Later on we might want to have config options to let the PD assume
other states of SDA/SCL initially.