[BLUETOOTH] sco: Possible double free.

this fixes coverity bug id #1068.
hci_send_sco() frees skb if (skb->len > hdev->sco_mtu).
Since it returns a negative error value only in this case, we
can directly return here.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>