iio: st_sensors: Disable DRDY at init time

This fixes odd behavior after reboot.

The fact that we set the device to powerdown mode is not sufficient to
prevent DRDY being active because we might still have an unread sample.

Even if powerdown was sufficient keeping DRDY disabled while trigger is
not active is a good idea.

Cc: Giuseppe Barba <giuseppe.barba@st.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>