ccid: Fix crash when backend isn't specified

Reproducer:

./x86_64-softmmu/qemu-system-x86_64 -device usb-ccid,id=ccid0 -usb -device ccid-card-emulated -monitor stdio

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
