i2c: add a warning to i2c_adapter_depth()

This routine is only used together with lockdep for nested locking.
The number of lock subclasses is limited to 8 as defined in lockdep.h

Emit a warning if the adapter depth exceeds the maximum number of
lockdep subclasses.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Peter Rosin <peda@axentia.se>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>