Bluetooth: Adjust L2CAP Max PDU size for AMP packets

Maximum PDU size is defined by new BT Spec as 1492 octets.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Reviewed-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>