iio:potentiostat:lmp91000: Drop of_match_ptr and use generic fw accessors

This change allows use of this driver with ACPI via PRP0001 and removes
an example of an anti pattern I'm trying to remove from IIO.

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/20200910173242.621168-17-jic23@kernel.org