iio:chemical:atlas-sensor: Drop of_match_ptr and use generic fw accessors

of_match_ptr() prevents use of this driver with ACPI via PRP0001 and is
an example of an anti pattern I'm trying to remove from IIO.
Hence drop from this driver and use generic fw accessors to check
if there is a fw_node and get the id.

It might be neater to use pointers rather than indexes for
the device_data but that is another issue and should be handled
separately.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20200910173242.621168-26-jic23@kernel.org