Bluetooth: Refactor locking in amp_physical_cfm

Remove locking from l2cap_physical_cfm and lock chan inside
amp_physical_cfm.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>