i2c: dev: Fix bus callback return values

The i2cdev_{at,de}tach_adapter() callbacks are used for two purposes:
  1. As notifier callbacks, when (un)registering I2C adapters created or
     destroyed after i2c_dev_init(),
  2. As bus iterator callbacks, for registering already existing
     adapters from i2c_dev_init(), and for cleanup.

Unfortunately both use cases expect different return values: the former
expects NOTIFY_* return codes, while the latter expects zero or error
codes, and aborts in case of error.

Hence in case 2, as soon as i2cdev_{at,de}tach_adapter() returns
(non-zero) NOTIFY_OK, the bus iterator aborts.  This causes (a) only the
first already existing adapter to be registered, leading to missing
/dev/i2c-* entries, and (b) a failure to unregister all but the first
I2C adapter during cleanup.

Fix this by introducing separate callbacks for the bus iterator,
wrapping the notifier functions, and always returning succes.
Any errors inside these callback functions are unlikely to happen, and
are fatal anyway.

Fixes: cddf70d0

 ("i2c: dev: fix notifier return values")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>