Since ACPI_PTR() is used to NULLify the value when !CONFIG_ACPI,
struct 'bma220_acpi_id' becomes defined but unused.

Fixes the following W=1 kernel build warning(s):

 drivers/iio/accel/bma220_spi.c:312:36: warning: ‘bma220_acpi_id’ defined but not used [-Wunused-const-variable=]
 312 | static const struct acpi_device_id bma220_acpi_id[] = {
 | ^~~~~~~~~~~~~~

Cc: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>