mmc: sdhci-esdhc-imx: fix the mask for tuning start point

According the RM, the bit[6~0] of register ESDHC_TUNING_CTRL is
TUNING_START_TAP, bit[7] of this register is to disable the command
CRC check for standard tuning. So fix it here.

Fixes: d87fc966

 ("mmc: sdhci-esdhc-imx: support setting tuning start point")
Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Link: https://lore.kernel.org/r/1590488522-9292-1-git-send-email-haibo.chen@nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>