mmc: sdhci-pltfm: Constify the ops field of sdhci_pltfm_data struct

All users of the sdhci_ops struct in the sdhci core already treat it as
const.  The sdhci-pltfm code itself never actually looks at the ops field
of the sdhci_pltfm_data struct and merely passes it on to the sdhci core,
so make we can make it const in the sdhci_pltfm_data struct as well.
This allows us to declare sdhci_ops structs as const in drivers using
the sdhci-pltfm helper code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>