mmc: core: Move mmc_card_removed() into mmc_start_request()

Both callers of mmc_start_request() call mmc_card_removed()
so move that call into mmc_start_request().

This patch is preparation for adding re-tuning support.

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