iio: st_sensors: make interrupt optional

Some sensors such as magnetometers and pressure sensors doesn't
have interrupts at all, and thus no DRDY setting applies. Make
the assignment of an interrupt optional, and do not call
st_sensors_set_drdy_int_pin() if there is no drdy (data ready)
pin specified.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>