iio: adc: ad7949: use spi_is_bpw_supported()

iio: adc: ad7949: use spi_is_bpw_supported()

Use spi_is_bpw_supported() instead of directly accessing spi->controller
->bits_per_word_mask. bits_per_word_mask may be 0, which implies that
8-bits-per-word is supported. spi_is_bpw_supported() takes this into
account while spi_ctrl_mask == SPI_BPW_MASK(8) does not.

Fixes: 0b2a740b424e ("iio: adc: ad7949: enable use with non 14/16-bit controllers")
Closes: https://lore.kernel.org/linux-spi/c8b8a963-6cef-4c9b-bfef-dab2b7bd0b0f@sirena.org.uk/
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20250611-iio-adc-ad7949-use-spi_is_bpw_supported-v1-1-c4e15bfd326e@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>