Bluetooth: btusb: Add btusb devcoredump support

This patch implements the btusb driver side .coredump() callback to
trigger a devcoredump via sysfs.

Signed-off-by: Manish Mandlik <mmandlik@google.com>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>