bearer-mbim: fix warnings with -Wimplicit-fallthrough

mm-bearer-mbim.c: In function ‘connect_context_step’:
  mm-bearer-mbim.c:843:18: error: this statement may fall through [-Werror=implicit-fallthrough=]
    843 |         ctx->step++;
        |         ~~~~~~~~~^~
  mm-bearer-mbim.c:845:5: note: here
    845 |     case CONNECT_STEP_PACKET_SERVICE: {
        |     ^~~~
  mm-bearer-mbim.c: In function ‘disconnect_context_step’:
  mm-bearer-mbim.c:1269:18: error: this statement may fall through [-Werror=implicit-fallthrough=]
   1269 |         ctx->step++;
        |         ~~~~~~~~~^~
  mm-bearer-mbim.c:1271:5: note: here
   1271 |     case DISCONNECT_STEP_DISCONNECT: {
        |     ^~~~

src/mm-bearer-mbim.c

@@ -839,8 +839,8 @@ connect_context_step (GTask *task)
 
     switch (ctx->step) {
     case CONNECT_STEP_FIRST:
-        /* Fall down */
         ctx->step++;
+        /* Fall through */
 
     case CONNECT_STEP_PACKET_SERVICE: {
         GError *error = NULL;
@@ -1268,8 +1268,8 @@ disconnect_context_step (GTask *task)
 
     switch (ctx->step) {
     case DISCONNECT_STEP_FIRST:
-        /* Fall down */
         ctx->step++;
+        /* Fall through */
 
     case DISCONNECT_STEP_DISCONNECT: {
         MbimMessage *message;