mmc: fsl_esdhc_imx: drop redundant clock settings

During mmc initialization, there are several calls to mmc_set_clock
and mmc_set_ios. When mmc_power_off, the mmc->clock will be set,
but the imx driver will use 400KHz. So the following calls
to mmc_set_ios will set the clock several times which is redundant
in fsl_esdhc_imx driver. So let's simplify to remove redundant
clock settings.

Signed-off-by: Peng Fan <peng.fan@nxp.com>