mmc: fsl_esdhc: Do not set high speed mode on MX25 and MX51

mmc: fsl_esdhc: Do not set high speed mode on MX25 and MX51

Currently when a high speed SD card is connected on MX25 or MX51 boards
the following error happens:

U-Boot 2017.11-rc2 (Oct 18 2017 - 13:49:26 -0200)

CPU:   Freescale i.MX51 rev3.0 at 800 MHz
Reset cause: POR
Board: MX51EVK
DRAM:  512 MiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
*** Warning - read failed, using default environment

In:    serial
Out:   serial
Err:   serial
Net:   FEC
Hit any key to stop autoboot:  0
=> saveenv
Saving Environment to MMC...
Writing to MMC(0)... failed

Workaround this issue by not setting the mmc high speed mode flags even
if the HOSTCAPBLT register reports that the SD card can operate at
high speed.

Tested on imx51evk and imx25pdk boards.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Otavio Salvador <otavio@ossystems.com.br> # imx25pdk
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>