quoted

I wonder if bailing out on an unknown flag shouldn't be revisited in
general? I mean this will happen again when a new I2C_M_* flag is
introduced.

If it's guaranteed that any new flags are optional to handle by the driver,
than that is certainly better. I'll post a v3 with that approach.

If there will be a new flag, it is highly likely that it will handle
some corner case which only gets applied when there is a I2C_FUNC_* flag
guarding it. If the new flag turns out to be mandatory, the (poor)
author needs to check with all existing drivers anyhow.