mmc: dw_mmc-starfive: Fix initialization of prev_err

mmc: dw_mmc-starfive: Fix initialization of prev_err

Fix a bug by making sure prev_err doesn't get used when being
uninitialized.

Signed-off-by: William Qiu <william.qiu@starfivetech.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Fixes: 9e622229bbf4 ("mmc: starfive: Add sdio/emmc driver support")
Link: https://lore.kernel.org/r/20230307024646.10216-3-william.qiu@starfivetech.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>