iio: adc: ad7944: Consolidate spi_sync() wrapper

iio: adc: ad7944: Consolidate spi_sync() wrapper

Since commit 6020ca4de8e5 ("iio: adc: ad7944: use spi_optimize_message()"),
The helper functions wrapping spi_sync() for 3-wire and 4-wire modes are
virtually identical. Since gpiod_set_value_cansleep() does a NULL check
internally, we can consolidate the two functions into one and avoid
switch statements at the call sites.

The default cases of the removed switch statement were just to make the
compiler happy and are not reachable since the mode is validated in the
probe function. So removing those should be safe.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240412-ad7944-consolidate-msg-v1-1-7fdeff89172f@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>