mmc: core: Use generic CMD6 time while switching to eMMC HS200 mode

Conform to the eMMC spec and use the CMD6 generic timeout from the
EXT_CSD register, when switching to HS200 mode.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>