mmc: sdhci-st: Do cleanup while mmc_of_parse() return an error

At errors, balance sdhci_pltfm_init() with sdhci_pltfm_free(), to make
sure all data is freed properly.

Cc: Srinivas Kandagatla <srinivas.kandagatla@gmail.com>
Cc: Maxime Coquelin <maxime.coquelin@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>