mmc: allow upper layers to know immediately if card has been removed

Add a function mmc_detect_card_removed() which upper layers can use to
determine immediately if a card has been removed. This function should
be called after an I/O request fails so that all queued I/O requests
can be errored out immediately instead of waiting for the card device
to be removed.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>