Bluetooth: mediatek: Fix memory leak

In case memory resources for *fw* were allocated, release them before
return.

Addresses-Coverity-ID: 1472611 ("Resource leak")
Fixes: 7237c4c9

 ("Bluetooth: mediatek: Add protocol support for MediaTek serial devices")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>