mmc: sdhci-st: Fix modalias to make module auto-loading work

Make the modalias match driver name, this is required to make module
auto-loading work.

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
CC: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>