mmc: tmio: make tmio_mmc_request function more readable

This part confused me and I had to read it twice until I got it. Let's
follow the standard pattern to bail out if something is wrong and keep
in the body of the function when everything is as expected.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>