mmc: host: tmio: don't BUG on unsupported stop commands

Halting the kernel on an unsupported stop command seems overkill, report
the error and say what we already did (due to autocmd12) instead.

Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>