Bluetooth: Remove holding lock when sending A2MP message

Remove holding the AMP manger list lock when sending the
A2MP Change Notify message, as this uses L2CAP and will
lock the socket and possibly sleep.

This removes a potential deadlock and a "BUG: sleeping
function called from invalid context" message from the
kernel log.

Change-Id: Icf8c5afc1a5cde36517bb979820c83a20ee207fd
CRs-fixed: 358736
Signed-off-by: Peter Krystad <pkrystad@codeaurora.org>