Cellular: unified return value comments on API folder.

Author: Teppo Järvelin

features/cellular/framework/API/CellularDevice.h

@@ -225,7 +225,11 @@ class CellularDevice {
      *  For example, when multiple modules are supported in a single AT driver this function detects
      *  and adapts to an actual module at runtime.
      *
-     *  @return 0 on success
+     *  @return         NSAPI_ERROR_OK on success
+     *                  NSAPI_ERROR_NO_MEMORY on case of memory failure
+     *                  NSAPI_ERROR_UNSUPPORTED if current model is not detected
+     *                  NSAPI_ERROR_DEVICE_ERROR if model information could not be read
+     *
      */
     virtual nsapi_error_t init_module() = 0;
 

features/cellular/framework/API/CellularInformation.h

@@ -80,7 +80,7 @@ class CellularInformation {
         IMEISV = 2, // IMEI and Software Version number
         SVN  = 3 // Software Version Number
     };
-    virtual nsapi_size_or_error_t get_serial_number(char *buf, size_t buf_size, SerialNumberType type = SN) = 0;
+    virtual nsapi_error_t get_serial_number(char *buf, size_t buf_size, SerialNumberType type = SN) = 0;
 };
 
 } // namespace mbed