Bluetooth: Remove sco_chan_get helper function

The sco_chan_get helper function is only used in two places and really
only protects conn->sk with a lock. So instead of hiding that fact,
just put the actual code in place where it is used.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>