iio:dac:ad5755: Switch to generic firmware properties and drop pdata

Lars pointed out that platform data can also be supported via the
generic properties interface, so there is no point in continuing to
support it separately.  Hence squish the linux/platform_data/ad5755.h
header into the c file and drop accessing the platform data directly.

Done by inspection only.  Mostly completely mechanical with the
exception of a few places where default value handling is
cleaner done by first setting the value, then calling the
firmware reading function but and not checking the return value,
as opposed to reading firmware then setting the default if an error
occurs.

Part of general attempt to move all of IIO over to generic
device properties, both to enable other firmware types and
to remove drivers that can be the source of of_ specific
behaviour in new drivers.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>