mmc: core: clear flags before allowing to retune

It might be that something goes wrong during tuning so the MMC core will
immediately trigger a retune. In our case it was:

 - we sent a tuning block
 - there was an error so we need to send an abort cmd to the eMMC
 - the abort cmd had a CRC error
 - retune was set by the MMC core

This lead to a vicious circle causing a performance regression of 75%.
So, clear retuning flags before we enable retuning to start with a known
cleared state.

Reported-by Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Fixes: bd11e8bd

 ("mmc: core: Flag re-tuning is needed on CRC errors")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210624151616.38770-2-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>