staging:iio:adt7410: use local platformdata if none is specified

The adt7410 expects information about an irq in platform_data.
The driver can work without an irq, so make platform_data optional
by specifying a dummy platform_data if the device has none.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>