iio: st_pressure: st_accel: Initialise sensor platform data properly

This patch fixes the sensor platform data initialisation for st_pressure
and st_accel device drivers. Without this patch, the driver fails to
register the sensors when the user removes and re-loads the driver.

1. Unload the kernel modules for st_pressure
$ sudo rmmod st_pressure_i2c
$ sudo rmmod st_pressure

2. Re-load the driver
$ sudo insmod st_pressure
$ sudo insmod st_pressure_i2c

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