unicore32: 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/unicore32/kernel/puv3-nb0916.c

@@ -54,6 +54,7 @@ static struct platform_pwm_backlight_data nb0916_backlight_data = {
 	.max_brightness	= 100,
 	.dft_brightness	= 100,
 	.pwm_period_ns	= 70 * 1024,
+	.enable_gpio	= -1,
 };
 
 static struct gpio_keys_button nb0916_gpio_keys[] = {