mmc: renesas_sdhi: initialize variable properly when tuning

'cmd_error' is not necessarily initialized on some error paths in
mmc_send_tuning(). Initialize it.

Fixes: 2c9017d0

 ("mmc: renesas_sdhi: abort tuning when timeout detected")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211130132309.18246-1-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>