mmc: sdhci: Stop exporting sdhci_send_command()

sdhci_send_command() has not been used outside of sdhci.c for many
years. Stop exporting it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/20200412090349.1607-3-adrian.hunter@intel.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>