iio: sensors-core: st: Support sensors which don't have a Data Ready pin

Not all ST's sensors support data ready, so let's make the declaration
of one conditional.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --cc drivers/iio/common/st_sensors/st_sensors_core.c
Simple merge

diff --cc drivers/iio/pressure/st_pressure_core.c
Simple merge