iio: imu: lsm6dsx: Fix mount matrix retrieval

The function lsm6dsx_get_acpi_mount_matrix should return an error when ACPI
support is not enabled to allow executing iio_read_mount_matrix in the
probe function.

Fixes: dc3d25f22b88 ("iio: imu: lsm6dsx: Add ACPI mount matrix retrieval")
Signed-off-by: Alejandro Tafalla <[e-mail]>
Acked-by: Lorenzo Bianconi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c

···
 static int lsm6dsx_get_acpi_mount_matrix(struct device *dev,
 					  struct iio_mount_matrix *orientation)
 {
-	return false;
+	return -EOPNOTSUPP;
 }
 
 #endif