hw/audio: Fix crashes when devices are used on ISA bus without DMA

The cs4231a, gus and sb16 sound cards crash QEMU when the user tries
to instantiate them on a machine with DMA-less ISA bus (for example
with "qemu-system-mips64el -M mips -device sb16"). Add proper checks
to the realize functions to avoid the crashes.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1521193892-15552-4-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>