mmc: sdhci-st: remove unnecessary platform_set_drvdata() call

sdhci_pltfm_init() has already set the platform drvdata for us, remove
the unnecessary platform_set_drvdata() call.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>