Bluetooth: L2CAP: Derive MPS from connection MTU

This ensures the MPS can fit in a single HCI fragment so each
segment don't have to be reassembled at HCI level, in addition to
that also remove the debugfs entry to configure the MPS.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>