mmc: core: Do not set host->card to NULL if oldcard is not released

When card initiated mmc_power_restore_host() which triggers 2nd time
mmc_sdio_init_card(), we free the new allocated card and keep oldcard,
so we cannot set host->card to NULL if we still have oldcard.

Change-Id: I50cf6b543244fe929bc08da64571b7ccfd734f77
CRs-Fixed: 2487134
Signed-off-by: hangtian <hangtian@codeaurora.org>