Bluetooth: Ignore BR/EDR packet size constraints when fragmenting for AMP

When operating over BR/EDR, ERTM accounts for the maximum over-the-air
packet size when setting the PDU size.  AMP controllers do not use the
same over-the-air packets, so the PDU size should only be based on the
HCI MTU of the AMP controller.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
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>