leds: ti-lmu-common: Fix coccinelle issue in TI LMU

Fix the coccinelle issues found in the TI LMU common code

drivers/leds/leds-ti-lmu-common.c:97:20-29: WARNING: Unsigned expression compared with zero: ramp_down < 0
drivers/leds/leds-ti-lmu-common.c:97:5-12: WARNING: Unsigned expression compared with zero: ramp_up < 0

Fixes: 3fce8e1e ("leds: TI LMU: Add common code for TI LMU devices")
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>