Bluetooth: ISO: Use kref to track lifetime of iso_conn

Bluetooth: ISO: Use kref to track lifetime of iso_conn

This make use of kref to keep track of reference of iso_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 iso_sock_set_timer on iso_sock_disconn
since at that point it is useless to set a timer as the sk will be freed
there is nothing to be done in iso_sock_timeout.

Fixes: ccf74f2390d6 ("Bluetooth: Add BTPROTO_ISO socket type")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>