bluetooth: cyw43xxx: support autobaud feature

Newer AIROC controllers like the CYW55573 don't support changing the
baudrate in Download Mode. However, a higher baud rate can be used
directly to sent HCI Reset.

This commit adds the KConfig flag CONFIG_AIROC_DOWNLOAD_MODE to enable
the new behaviour.

Signed-off-by: Matthias Ringwald <matthias@ringwald.ch>