Bluetooth: Fix busy return for mgmt_set_powered in some cases

We should return a "busy" error always when there is another
mgmt_set_powered operation in progress. Previously when powering on
while the auto off timer was still set the code could have let two or
more pending power on commands to be queued. This patch fixes the issue
by moving the check for duplicate commands to an earlier point in the
set_powered handler.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>