hw/sd: Don't pass BlockBackend to sd_reset()

The only valid BlockBackend to pass to sd_reset() is the one for
the SD card, which is sd->blk. Drop the second argument from this
function in favour of having it just use sd->blk.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1430683444-9797-1-git-send-email-peter.maydell@linaro.org
