Bluetooth: btusb: QCA: Support downloading custom-made firmwares

There are custom-made firmwares based on board ID for a given QCA BT
chip sometimes, and they are different with existing firmwares and put
in a separate subdirectory to avoid conflict, for example:
QCA2066, as a variant of WCN6855, has firmwares under 'qca/QCA2066/'
of linux-firmware repository.

Support downloading custom-made firmwares based on a table newly added.

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