driver core: bus.h: document bus notifiers better

driver core: bus.h: document bus notifiers better

The bus notifier values are not documented all that well, so clean this
up and make a real enumerated type for them and document them much
better.  When doing this, remove the hex values and just rely on the
enumerated type instead as that is all that is needed.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230111092331.3946745-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>