mmc: core: Only change mode if mmc_select_bus_width() is successful

mmc_select_bus_width() returns bus width (4 or 8) on success or
zero if unsupported. So only change mode if setting the bus width
is successful.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>