Bluetooth: Use explicit AMP controller id value for BR/EDR

The special AMP controller id 0 is reserved for the BR/EDR controller
that has the main link. It is a fixed value and so use a constant for
this throughout the code to make it more visible when the handling is
for the BR/EDR channel or when it is for the AMP channel.

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