backlight: tps65217_bl: Remove unnecessary default brightness check

Variable val holds the default brightness and is unsigned, therefore
checking whether it is less than zero is redundant.

Signed-off-by: Christos Gkekas <chris.gekas@gmail.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>