Bluetooth: SCO: Use kref to track lifetime of sco_conn

This make use of kref to keep track of reference of sco_conn which
allows better tracking of its lifetime with usage of things like
kref_get_unless_zero in a similar way as used in l2cap_chan.

In addition to it remove call to sco_sock_set_timer on __sco_sock_close
since at that point it is useless to set a timer as the sk will be freed
there is nothing to be done in sco_sock_timeout.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>