sdcard: fix the 'maximum data transfer rate' to 25MHz

sdcard: fix the 'maximum data transfer rate' to 25MHz

To comply with Spec v1.10 (and 2.00, 3.01):

. TRAN_SPEED

for current SD Memory Cards that field must be always 0_0110_010b (032h) which is
equal to 25MHz - the mandatory maximum operating frequency of SD Memory Card.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180215221325.7611-4-f4bug@amsat.org
[PMM: fixed comment indent]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>