Bluetooth: Remove useless external function to count controllers

The list of controllers can be counted ahead of time and inline
inside the AMP discover handling. There is no need to export such
a function at all.

In addition just count the AMP controller and only allocated space
for a single mandatory BR/EDR controller. No need to allocate more
space than needed.

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