Bluetooth: Remove check for number of AMP controller

The number of controllers for the AMP discover response has already
been calculated. And since the hci_dev_list lock is held, it can not
change. So there is no need for any extra checks.

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