ads7846: put it into the 'input' category

The category of the ads7846 device is not set, put it into the 'input'
category.

Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20201112125824.763182-3-ganqixin@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/display/ads7846.c

@@ -163,10 +163,12 @@ static void ads7846_realize(SSISlave *d, Error **errp)
 
 static void ads7846_class_init(ObjectClass *klass, void *data)
 {
+    DeviceClass *dc = DEVICE_CLASS(klass);
     SSISlaveClass *k = SSI_SLAVE_CLASS(klass);
 
     k->realize = ads7846_realize;
     k->transfer = ads7846_transfer;
+    set_bit(DEVICE_CATEGORY_INPUT, dc->categories);
 }
 
 static const TypeInfo ads7846_info = {