iio:dac:mcp4725: drop of_match_ptr and use generic fw properties

This enables use of ACPI PRP0001 and removes an antipattern I am
trying to stop people copying in IIO.

This particular case is more complex than most because it allowed
probing via sysfs with out a fwnode but would presumably always
have then failed.   Now the code will assume that properties are
the defaults if not specified or the firmware node is not present.
This relaxation of the constraints should not break any existing
cases and may enable some new ones.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Link: https://lore.kernel.org/r/20200910173242.621168-14-jic23@kernel.org