i2c: Replace list-based mechanism for handling userspace-created clients

Similar to the list of auto-detected clients, we can also replace the
list of userspace-created clients with flagging such client devices.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
[wsa: fixed description of the new flag; reordered new code in
'device_store' to have single exit point; fixed whitespace errors;
 folded cleanup patch into this one]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>