Bluetooth: Add constants for SCO airmode

This patch defines constants for SCO airmode from SCO voice setting. It
refers to Bluetooth Core V4.0 specification, Part E, Chap 6.12 which
describe SCO voice setting format.

Signed-off-by: Frédéric Dalleau <frederic.dalleau@linux.intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>