[control] Make internal module state data static

Global data is problematic in general and these values have accessor
functions anyway and thus there is no reason for extern visibility.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>

src/usb_moded-control.c

@@ -62,21 +62,21 @@ bool           control_get_connection_state         (void);
  *
  * What was the last mode signaled over D-Bus.
  */
-char *control_external_mode = NULL;
+static char *control_external_mode = NULL;
 
 /** The logical mode name
  *
  * Full set of valid modes can occur here
  */
-char *control_internal_mode = NULL;
+static char *control_internal_mode = NULL;
 
 /** Connection status
  *
  * Access only via:
  * - control_set_cable_state()
  * - control_get_connection_state()
  */
-cable_state_t control_cable_state = CABLE_STATE_UNKNOWN;
+static cable_state_t control_cable_state = CABLE_STATE_UNKNOWN;
 
 /* ========================================================================= *
  * Functions