Only set indio_dev->label from of/dt if there actually is a label
specified in of.

This allows drivers to set a label without this being overwritten with
NULL when there is no label specified in of. This is esp. useful on
devices where of is not used at all, such as your typical x86/ACPI device.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210207160901.110643-2-hdegoede@redhat.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>