i2c: Warn when device removing fails

The driver core ignores the return value of struct bus_type::remove. So
warn if there is an error that went unnoticed before and return 0
unconditionally in i2c_device_remove().

This prepares changing struct bus_type::remove to return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[wsa: added a comment and removed unneeded initializtion]
Signed-off-by: Wolfram Sang <wsa@kernel.org>