accel: Fix getting scale for sensors with IIO_SHARED_BY_ALL mask

Properties with the IIO_SHARED_BY_ALL mask don't have the sensor type prefix
before the filename, causing some devices to not have a scale set from sysfs.

Fix this by searching through sysfs for both properties name: scale and
in_accel_scale.

Tested on: Cyan (Acer Chromebook R11, with 2 accelerometers)
Affects: All chromebooks with cros-ec sensors
https://github.com/torvalds/linux/tree/master/drivers/iio/common/cros_ec_sensors

src/accel-attributes.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2019 Luís Ferreira <luis@aurorafoss.org>
+ * Copyright (c) 2019 Daniel Stuart <daniel.stuart@pucpr.edu.br>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 3 as published by
+ * the Free Software Foundation.
+ *
+ */
+
+#include <glib.h>
+#include <gudev/gudev.h>
+
+typedef enum {
+	ACCEL_LOCATION_DISPLAY,
+	ACCEL_LOCATION_BASE,
+} AccelLocation;
+
+AccelLocation setup_accel_location (GUdevDevice *device);
+
+gboolean parse_accel_location (const char    *location,
+                               AccelLocation *value);
+
+gdouble get_accel_scale (GUdevDevice *device);