[bt][rfcomm] fix style nit

C-style casts are passé. More importantly, though, they're hard to
grep for. Remove the legacy cast that I noticed.

Bug: None
Test: build

Change-Id: Ia58fe21927d2734a1681e1a855ed92aa63716dc6
diff --git a/garnet/drivers/bluetooth/lib/rfcomm/session.cc b/garnet/drivers/bluetooth/lib/rfcomm/session.cc
index 42b2544..89d5d8b 100644
--- a/garnet/drivers/bluetooth/lib/rfcomm/session.cc
+++ b/garnet/drivers/bluetooth/lib/rfcomm/session.cc
@@ -242,8 +242,7 @@
   }
 
   DLCI dlci = frame->dlci();
-
-  switch ((FrameType)frame->control()) {
+  switch (static_cast<FrameType>(frame->control())) {
     case FrameType::kSetAsynchronousBalancedMode:
       HandleSABM(dlci);
       return;