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>