mmc: sdhci: Use Auto CMD Auto Select only when v4_mode is true

mmc: sdhci: Use Auto CMD Auto Select only when v4_mode is true

sdhci-of-dwcmshc meets an eMMC read performance regression with below
command after commit 427b6514d095 ("mmc: sdhci: Add Auto CMD Auto
Select support"):

dd if=/dev/mmcblk0 of=/dev/null bs=8192 count=100000

Before the commit, the above command gives 120MB/s
After the commit, the above command gives 51.3 MB/s

So it looks like sdhci-of-dwcmshc expects Version 4 Mode for Auto
CMD Auto Select. Fix the performance degradation by ensuring v4_mode
is true to use Auto CMD Auto Select.

Fixes: 427b6514d095 ("mmc: sdhci: Add Auto CMD Auto Select support")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201015174115.4cf2c19a@xhacker.debian
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>