Bluetooth: Fix warning of potentially uninitialized adv_instance variable

Rework the logic of checking for a valid adv_instance for non-zero
cp->instance values. Without this change we may get (false positive)
warnings as follows:

>> net/bluetooth/mgmt.c:7294:29: warning: 'adv_instance' may be used
uninitialized in this function [-Wuninitialized]

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