mmc: core: fold mmc_set_bus_width calls into sdio_enable_4bit_bus.

Every call to sdio_enable_4bit_bus is followed (on success) by a call
to mmc_set_bus_width().

To simplify the code, include those calls directly in
sdio_enable_4bit_bus().

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>