leds: da903x: (da9030 only) led brightness reversed.

The brightness control register calculation (for the pwm) is
effectively the reverse of what would be expected.
1 is maximum brightness, 255 minimum.

This patch inverts this.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Eric Miao <eric.miao@marvell.com>