staging: greybus: fix get_channel_from_mode() failure path

[ Upstream commit 34164202 ]

The get_channel_from_mode() function is supposed to return the channel
which matches the mode.  But it has a bug where if it doesn't find a
matching channel then it returns the last channel.  It should return
NULL instead.

Also remove an unnecessary NULL check on "channel".

Fixes: 2870b52b

 ("greybus: lights: add lights implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/379c0cb4-39e0-4293-8a18-c7b1298e5420@moroto.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>