greybus: lights: check return of get_channel_from_mode

greybus: lights: check return of get_channel_from_mode

If channel for the given node is not found we return null from
get_channel_from_mode. Make sure we validate the return pointer
before using it in two of the missing places.

This was originally reported in [0]:
Found by Linux Verification Center (linuxtesting.org) with SVACE.

[0] https://lore.kernel.org/all/20240301190425.120605-1-m.lobanov@rosalinux.ru

Fixes: 2870b52bae4c ("greybus: lights: add lights implementation")
Reported-by: Mikhail Lobanov <m.lobanov@rosalinux.ru>
Suggested-by: Mikhail Lobanov <m.lobanov@rosalinux.ru>
Suggested-by: Alex Elder <elder@ieee.org>
Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Link: https://lore.kernel.org/r/20240325221549.2185265-1-rmfrfs@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>