staging: iio: resolver: ad2s1210: use regmap for config registers

This makes use of the regmap API to read and write the configuration
registers. This simplifies code quite a bit and makes it safer
(previously, it was easy to write a bad value to the config registers
which causes the chip to lock up and need to be reset).

This chip has multiple modes of operation. In normal mode, we do not use
regmap since there is no addressing - data is just bitshifted out during
the SPI read. In config mode, we use regmap since it requires writing
the address (with read/write flag) before reading and writing.

We don't use the lock provided by the regmap because we need to also
synchronize with the normal mode SPI reads and with the various GPIOs.

There is also a quirk when reading registers (other than the fault
register). If the address/data bit is set in the value read, then it
indicates there is a configuration parity error and the data is not
valid. Previously, this was checked in a few places, but not
consistently. Now, we always check it in the regmap read function.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-10-fa4364281745@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>