Bluetooth: ATT: Fix bogus CONTAINER_OF() usage

This CONTAINER_OF() worked by chance, but was actually wrong, since it
makes it look like bt_l2cap_le_chan is the parent container of bt_att.
Instead bt_l2cap_le_chan is simply a member of bt_att, i.e. we can
dereference it directly.

Change-Id: I7307517bae823e54b45db31f75462655ce6eb50d
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>