eeprom: at24: provide and use a helper for releasing dummy i2c clients

This allows us to drop two opencoded for loops. We also don't need to
check if the i2c client is NULL before calling i2c_unregister_device().

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Peter Rosin <peda@axentia.se>