MLK-20957-1 mmc: sdhci-esdhc-imx: correct the fix of ERR004536

the commit 885c943ca13d ("ENGR00288842 mmc: sdhci-esdhc-imx: add
ADMA Length Mismatch errata fix") involve the fix of ERR004536.
But double confirm with IC, need to clear the bit 7 of register
0x6c rather than set this bit 7.
here is the function of bit 7 of 0x6c:
0: enable the new IC fix for ERR004536
1: do not use the IC fix, keep the same as before

Due to the reset value of this bit 7 is 0, and ROM code also do not
touch this bit 7, so this patch directly remove the operation of this
bit 7, make sure the fix of ERR004536 can work.

Note, for all versons of 6DQP 6DQ 6DL and 6SL, IC do not has this
hardware fix, so writing this bit has no effect and we keep using
ADMA as before which has been used for several years with the
consideration of no performance drop.

For other SoC like imx6SLL imx6SX imx6UL/imx6ULL imx7 imx8, IC already
contain this hareware fix, so must make sure the bit 7 of the register
0x6c is 0. If not, we meet the ADMA length mismatch error on imx8mm-evk
and imx8qxp-ddr3l-val board when enable CMDQ.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>