drivers/adcxx1c: fix SAUL return value in case of read error

drivers/adcxx1c/adcxx1c_saul.c

@@ -26,7 +26,9 @@
 
 static int read_adc(const void *dev, phydat_t *res)
 {
-    adcxx1c_read_raw((const adcxx1c_t *)dev, res->val);
+    if (adcxx1c_read_raw((const adcxx1c_t *)dev, res->val)) {
+        return -ECANCELED;
+    }
 
     res->unit = UNIT_NONE;
     res->scale = 0;