iio: magnetometer: rm3100-spi: remove bits_per_word = 8

Remove setting bits_per_word = 8 from the rm3100 driver. This is the
default value for SPI transfers, so it is not necessary to explicitly
set it.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250505-iio-remove-bits_per_word-8-v1-11-341f85fcfe11@baylibre.com


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