leds: tca6507: silence an uninitialized variable warning

If choose_times() returns -EINVAL that means "c1" and "c2" haven't been
initialized.  I've added a check for that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>