i2c: add helper to check if a client has a driver attached

As a preparation for an API conversion, factor out something frequently
used in the media subsystem. As an improvement, it bails out on both,
NULL and ERRPTR to handle the old and new API.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>