Bluetooth: Make amp_mgr_list and amp_mgr_list_lock static

There is no reason to have amp_mgr_list and amp_mgr_list_lock exported
from a2mp.c and thus make both of them static.

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