iio: magnetometer: st_magn_core: enable multiread by default for LIS3MDL

Enable multiread by default for LIS3MDL since using I2C interface
the subaddr Msb is used to automatically increase the slave address
during multiple data read/write. In the same way, using SPI interface,
bit 1 in register address is used to enable auto-increment of the slave
address in multiple read/write.

Fixes: 872e79ad (iio:magn: Add STMicroelectronics magn driver)
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>