backlight: fix memory leak on obscure error path

Dredged out of bugzilla

Reported-by: Martlin Ettl <ettl.martin@gmx.de>
Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=15492


Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>