greybus: lights: check return of get_channel_from_mode

In the Linux kernel, the following vulnerability has been resolved:

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/[email protected]