Bluetooth: Extend state_change() call to report errors too

Instead of creating an new function pointer to report errors we are just
reusing state_change for that and there is a simple reason for this, one
place in the l2cap_core.c code needs, in a locked sk, set both the sk_state
and sk_err. If we create two different functions for this we would need to
release the lock between the two operation putting the socket in non
desired state.

The change is transparent to the l2cap_core.c code, user that only needs
to set the state won't need any modification.

This is another step of an ongoing work to make l2cap_core.c totally
independent from l2cap's struct sock.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>