bt-l2cap: fix if statement with empty body, spotted by clang

bt-l2cap: fix if statement with empty body, spotted by clang

Fix clang error:
  CC    bt-l2cap.o
/src/qemu/hw/bt-l2cap.c:1000:41: error: if statement has empty body
[-Wempty-body]
            /* TODO: Signal an error? */;

This means that l2cap_sframe_in() may now get called.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Andrew Zaborowski <balrogg@gmail.com>