mmc: sdhci-esdhc-imx: clear tuning bits during driver probe

The tuning bits like FBCLK_SEL, SMP_CLK_SEL and DLY_CELL which affects
timing may have already been set by ROM if booting from SD3.0 mode like
SDR104. Let's clear it first during driver probe before doing the new
card enumeration to avoid working on the wrong timing.

Note that tuning bits are dynamical settings which may need to be kept
during MMC_PM_KEEP_POWER suspend, so we did not put them into hwinit
function.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>