Bluetooth: Adds set_default function in L2CAP setup

Some parameters in L2CAP chan are set to default similar way in
socket based channels and A2MP channels. Adds common function which
sets all defaults.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>