generic-sdhci: Remove user_creatable flag

generic-sdhci needs to be wired by other devices' code, so it
can't be used with -device. Remove the user_creatable flag from
the device class.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Alexander Graf <agraf@suse.de>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Prasad J Pandit <pjp@fedoraproject.org>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-17-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>