mmc: dw_mmc: Don't start commands while busy

We've seen problems on some WiFi modules where we seem to send a CMD53
(which requires the data lines) while the module is asserting busy.
We shouldn't do that.

The Designware Databook says that before issuing a new data transfer
command we should check for busy, so that's what we'll do.

We'll leverage the existing dw_mmc knowledge about whether it should
wait for the previous command to finish to know whether we should
check for busy before sending the command.  This means we won't end up
incorrectly waiting for things like CMD52 (SDIO) or CMD13 (SD) which
don't use the data line.

Note that this also has the advantage of making sure that we don't
change the clock while the card is busy, too.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>