iio: st_accel: Fix unused variable warning

drivers/iio/accel/st_accel_core.c:1005:44: warning:
 mount_matrix_ext_info defined but not used [-Wunused-const-variable=]

Using stub helper while CONFIG_ACPI is disabled to fix it.

Suggested-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>