ARM: pxa: 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.

A special case is the Palm Tungsten|C board. Since it doesn't use any
quirks that would require the existing .init() or .exit() hooks it can
simply use the new enable_gpio field.

Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arm/mach-pxa/hx4700.c

@@ -561,6 +561,7 @@ static struct platform_pwm_backlight_data backlight_data = {
 	.max_brightness = 200,
 	.dft_brightness = 100,
 	.pwm_period_ns  = 30923,
+	.enable_gpio    = -1,
 };
 
 static struct platform_device backlight = {