staging: iio: resolver: ad2s1210: read excitation frequency from control register

This modifies the ad2s1210_show_fexcit() function to read the excitation
frequency from the control register. This way we don't have to keep
track of the value and don't risk returning a stale value.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-16-fa4364281745@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>