Bluetooth: btusb: Reduce a redundant assignment in btusb_probe()

Initialize @priv_size at declaration rather than separately:
- Simpler: one statement completes both declaration and assignment.
- More flexible: the variable is immediately usable from that point,
  so any new priv_size += can be freely inserted without caring about
  where the separate priv_size = 0 sits.

Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>