iio: accel: kionix-kx022a: Add a function to retrieve number of bytes in buffer

iio: accel: kionix-kx022a: Add a function to retrieve number of bytes in buffer

Since Kionix accelerometers use various numbers of bits to report data, a
device-specific function is required.
Implement the function as a callback in the device-specific chip_info structure

Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Mehdi Djait <mehdi.djait.k@gmail.com>
Link: https://lore.kernel.org/r/9c550fb28e34915d473e379f812c7753f7643bae.1694867379.git.mehdi.djait.k@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>