i2c: dev: fix notifier return values

We have a set of return values that notifier callbacks can return. They
should not return 0, error codes or anything other than those predefined
values. Make the i2c character device's callback return NOTIFY_DONE or
NOTIFY_OK depending on the situation.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>