mmc: sdhci: constify references of parameters to __sdhci_read_caps()

__sdhci_read_caps() does not modify *ver, *caps, or *caps1.

Probably, the caller of this function will want to constifythe
parameters passed in.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>