Bluetooth: btbcm: Factor out common code to determine subversion

We are using the same loop in both the UART and USB bus cases, refactor
things a bit to share the loop.

This is mostly meant to improve readability.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>