iio: adc: ad7944: drop bits_per_word hack

Remove setting bits_per_word in SPI xfers without data. The shortcoming
that this was working around was fixed in the SPI controller driver, so
it is no longer necessary. And we don't need this to be cargo-culted to
new drivers.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250331-iio-adc-ad7944-drop-bits_per_word-hack-v1-1-2b952e033340@baylibre.com


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