iface-modem-signal: fix warnings with -Wimplicit-fallthrough

mm-iface-modem-signal.c:447:18: error: this statement may fall through [-Werror=implicit-fallthrough=]
    447 |         ctx->step++;
        |         ~~~~~~~~~^~
  mm-iface-modem-signal.c:449:5: note: here
    449 |     case INITIALIZATION_STEP_CHECK_SUPPORT:
        |     ^~~~
  ...

src/mm-iface-modem-signal.c

@@ -443,8 +443,8 @@ interface_initialization_step (GTask *task)
             supported_quark = (g_quark_from_static_string (
                                    SUPPORTED_TAG));
 
-        /* Fall down to next step */
         ctx->step++;
+        /* fall through */
 
     case INITIALIZATION_STEP_CHECK_SUPPORT:
         if (!GPOINTER_TO_UINT (g_object_get_qdata (G_OBJECT (self),
@@ -470,8 +470,8 @@ interface_initialization_step (GTask *task)
             /* If there is no implementation to check support, assume we DON'T
              * support it. */
         }
-        /* Fall down to next step */
         ctx->step++;
+        /* fall through */
 
     case INITIALIZATION_STEP_FAIL_IF_UNSUPPORTED:
         if (!GPOINTER_TO_UINT (g_object_get_qdata (G_OBJECT (self),
@@ -483,8 +483,8 @@ interface_initialization_step (GTask *task)
             g_object_unref (task);
             return;
         }
-        /* Fall down to next step */
         ctx->step++;
+        /* fall through */
 
     case INITIALIZATION_STEP_LAST:
         /* We are done without errors! */