Bluetooth: Add convenience function to check for pending power off

There are several situations where we're interested in knowing whether
we're currently in the process of powering off an adapter. This patch
adds a convenience function for the purpose and makes it public since
we'll soon need to access it from hci_event.c as well.

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