mmc: tmio: Extract bus_width modifications to a separate function

Move code for bus_width modification, out of the ->set_ios() callback
and into a separate function, to simplify code.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>