SD : Updated CMD 6 implememtation.

SD : Updated CMD 6 implememtation.

For setting high speed in SD card,
First CMD 6 (Switch) is send to check if card supports High Speed and
Second command is send to switch card to high speed mode.

In current inplementation, CMD 6 was sent only once to switch the
card into HS mode without checking if card supports HS or not, which is
not as per specification and also we are not setting the HS i.e. 50000000
but directly asking the card to switch to 26000000 which is incorrect as
SD card supports either 25000000 or 50000000.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Meenakshi Aggarwal <meenakshi.aggarwal@nxp.com>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>