Bluetooth: Zero bredr pointer when chan is deleted

If BREDR L2CAP chan is deleted and this chan is the channel through
which High Speed traffic is routed to AMP then zero pointer to
the chan in amp_mgr to prevent accessing it.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>