at-serial-port: define flags by shifting bits

Otherwise glib-mkenums doesn't take the enumeration as being flags.

src/mm-at-serial-port.h

@@ -41,13 +41,13 @@ typedef struct _MMAtSerialPortClass MMAtSerialPortClass;
  *    only when connecting is port 0 opened for dialing (ATD) and PPP
  */
 typedef enum { /*< underscore_name=mm_at_port_flag >*/
-    MM_AT_PORT_FLAG_NONE      = 0x0000,
+    MM_AT_PORT_FLAG_NONE      = 0,
     /* This port is preferred for command and status */
-    MM_AT_PORT_FLAG_PRIMARY   = 0x0001,
+    MM_AT_PORT_FLAG_PRIMARY   = 1 << 0,
     /* Use port for command and status if the primary port is connected */
-    MM_AT_PORT_FLAG_SECONDARY = 0x0002,
+    MM_AT_PORT_FLAG_SECONDARY = 1 << 1,
     /* This port should be used for PPP */
-    MM_AT_PORT_FLAG_PPP       = 0x0004
+    MM_AT_PORT_FLAG_PPP       = 1 << 2,
 } MMAtPortFlag;
 
 typedef gboolean (*MMAtSerialResponseParserFn) (gpointer user_data,