leds: tca6507: Remove unnecessary reg check

Variable reg is unsigned so checking whether it is less than zero is not
necessary.

Signed-off-by: Christos Gkekas <chris.gekas@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>