Bluetooth: Check if mode is supported on getsockopt

Add this check to getsockopt makes possible to fail early instead of
waiting until listen / connect.

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