staging: iio: resolver: ad2s1210: refactor setting excitation frequency

staging: iio: resolver: ad2s1210: refactor setting excitation frequency

This combines the ad2s1210_update_frequency_control_word() and
ad2s1210_soft_reset() functions into a single function since they
both have to be called together. (The software reset does not reset
any configuration registers, it only updates the excitation output
and resets the tracking loop.)

Also clean up a few things while touching this:
- move AD2S1210_DEF_EXCIT macro with similar macros
- remove unnecessary dev_err() calls

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-15-fa4364281745@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>