greybus: fix pointless null check

Coccinelle points out that a call in gb_lights_channel_free() to
flush_work() is passed which is always non-null.   Prior to the
call, there is an unnecessary check to see if that address is null.
Get rid of the test.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>