Bluetooth: Allow setting of L2CAP ERTM via socket option

To enable Enhanced Retransmission mode it needs to be set via a socket
option. A different mode can be set on a socket, but on listen() and
connect() the mode is checked and ERTM is only allowed if it is enabled
via the module parameter.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>