ARM: shmobile: 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.
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arm/mach-shmobile/board-armadillo800eva.c

@@ -423,6 +423,7 @@ static struct platform_pwm_backlight_data pwm_backlight_data = {
 	.max_brightness = 255,
 	.dft_brightness = 255,
 	.pwm_period_ns = 33333, /* 30kHz */
+	.enable_gpio = -1,
 };
 
 static struct platform_device pwm_backlight_device = {