Bluetooth: Track the current configured random address

For Bluetooth controllers with LE support, track the value of the
currently configured random address. It is important to know what
the current random address is to avoid unneeded attempts to set
a new address. This will become important when introducing the
LE privacy support in the future.

In addition expose the current configured random address via
debugfs for debugging purposes.

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