mmc: core: stop trying to switch width when only one bit is supported

mmc_select_bus_width() will try to switch to MMC_BUS_WIDTH_4 even if
MMC_CAP_4_BIT_DATA and MMC_CAP_8_BIT_DATA are not set in host->caps.
Return as soon as possible when those flags are not set

Fixes: 577fb131 (mmc: rework selection of bus speed mode)
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: <stable@vger.kernel.org> # 3.17
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>