aspeed/smc: handle dummies only in fast read mode

HW works fine in normal read mode with dummy bytes being set. So let's
check this case to not transfer bytes.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1486648058-520-4-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>