ARM: OMAP: Initialize PWM backlight enable_gpio field

The GPIO API defines 0 as being a valid GPIO number, so this field needs
to be initialized explicitly.
Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arm/mach-omap2/board-zoom-peripherals.c

@@ -227,6 +227,7 @@ static struct platform_pwm_backlight_data zoom_backlight_data = {
 	.max_brightness = 127,
 	.dft_brightness = 127,
 	.pwm_period_ns = 7812500,
+	.enable_gpio = -1,
 };
 
 static struct platform_device zoom_backlight_pwm = {