i2c: Replace list-based mechanism for handling auto-detected clients

So far a list is used to track auto-detected clients per driver.
The same functionality can be achieved much simpler by flagging
auto-detected clients.

Two notes regarding the usage of driver_for_each_device:
In our case it can't fail, however the function is annotated __must_check.
So a little workaround is needed to avoid a compiler warning.
Then we may remove nodes from the list over which we iterate.
This is safe, see the explanation at the beginning of lib/klist.c.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
[wsa: fixed description of the new flag]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>