iio: st_sensors: handle memory allocation failure to fix null pointer dereference

A null pointer deference on pdata can occur if the allocation of
pdata fails.  Fix this by adding a null pointer check and handle
the -ENOMEM failure in the caller.

Addresses-Coverity: ("Dereference null return value")
Fixes: 3ce85cc4 ("iio: st_sensors: get platform data from device tree")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>