usb-bus: use snprintf

Avoid this warning from OpenBSD linker:
  LINK  i386-softmmu/qemu
../usb-bus.o(.text+0x27c): In function `usb_get_fw_dev_path':
/src/qemu/hw/usb-bus.c:294: warning: sprintf() is often misused,
please use snprintf()

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
