mmc: Do not poll using CMD13 when changing timing

When using CMD6 to switch eMMC card timing from HS200/HS400 to HS/legacy,
do not poll for the completion status using CMD13, but rather wait 50mS.

Once the card receives the CMD6 and starts executing it, the bus is in
undefined state until both the card finishes executing the command and
until the controller switches the bus to matching timing configuration.
During this time, it is not possible to transport any commands or data
across the bus, which includes the CMD13.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>