iio: chemical: bme680: Add read buffers in read/write buffer union

iio: chemical: bme680: Add read buffers in read/write buffer union

Move the buffers that are used in order to read data from the device in
the union which handles all the device read/write buffers. Also create
defines for the number of bytes that are being read from the device and
don't use magic numbers.

Signed-off-by: Vasileios Amoiridis <vassilisamir@gmail.com>
Link: https://patch.msgid.link/20240609233826.330516-10-vassilisamir@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>