Bluetooth: Make hci_send_sco() void

It also removes an unneeded check for the MTU. The check is done before
on sco_send_frame()

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>