Bluetooth: btusb: Fix memory leak

This checks if CONFIG_DEV_COREDUMP is enabled before attempting to clone
the skb and also make sure btmtk_process_coredump frees the skb passed
following the same logic.

Fixes: 0b701513 ("Bluetooth: btusb: mediatek: add MediaTek devcoredump support")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>