mmc: core: Update the ext-csd.rev check for eMMC5.1

With the new eMMC5.1 spec, there is a new EXT_CSD register with
the revision number(EXT_CSD_REV) 7. This patch updates the check
for ext-csd.rev number as 7.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Yuvaraj Kumar C D <yuvaraj.cd@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Chris Ball <cjb@laptop.org>