leds: lm3692x: Use flags from LM3692X_BRT_CTRL

Use LM3692X_RAMP_EN instead of LM3692X_PWM_HYSTER_4LSB
since the later is a flag for the PWM register. The
actual register value remains unchanged.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>