Bluetooth: btusb: Default CONFIG_BT_HCIBTUSB_POLL_SYNC=y

poll_sync has been proven to fix races of USB data and event endpoints
so this enables it by default.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Tedd Ho-Jeong An <tedd.an@intel.com>