Bluetooth: Create struct l2cap_chan

struct l2cap_chan cames to create a clear separation between what
properties and data belongs to the L2CAP channel and what belongs to the
socket. By now we just fold the struct sock * in struct l2cap_chan as all
the channel info is struct l2cap_pinfo today.

In the next commits we will see a move of channel stuff to struct
l2cap_chan.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>