leds: trigger: netdev: Add core support for hw not supporting fallback to LED sw control

If hw doesn't support sw control of the LED and we switch to a mode
not supported by hw, currently we get lots of errors because neither
brigthness_set() nor brithness_set_blocking() is set.
Deal with this by not falling back to sw control, and return
-EOPNOTSUPP to the user. Note that we still store the new mode.
This is needed in case an intermediate unsupported mode is necessary
to switch from one supported mode to another.

Add a comment explaining how a driver for such hw is supposed to behave.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/3fd5184c-3641-4b0b-b59a-f489ec69a6cd@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>