mmc: sdhci: Fix error paths in sdhci_add_host()

Some error paths in sdhci_add_host() simply returned without
cleaning up.  Also the return value from mmc_add_host()
was not being checked.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>