iio:magnetometer: Replace magic number 0 by IIO_CHAN_INFO_RAW

The function magn_3d_read_raw has a switch statement handling multiple
cases per channel. The first case statement uses the magic number 0,
which means IIO_CHAN_INFO_RAW. Additionally, the iio_chan_spec for
magn_3d_channels is configured to be IIO_CHAN_INFO_RAW. Therefore, this
patch replaces the magic number 0 for the appropriate IIO_CHAN_INFO_RAW.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>