Bluetooth: L2CAP: Initiate MTU configuration request

Triggers conditionally outgoing configuration request to start exchange
MTU just after incoming PSM connection request was aknowledged by local
host. If local MTU differs from default (672) add MTU option in request,
otherwise send empty configuration meaning default is wanted.

Change-Id: I4eb382cc496345c051967640422e926f595ce854
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>