Bluetooth: mgmt: Track pending class changes

This patch adds a flag to track pending changes to the class of device.
This is needed since we cannot cleanly handle multiple simultaneous
commands and need to return a "busy" error status in the mgmt commands
that might trigger a class change.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>