The main reason to add this check is to avoid unnecessary
mmc_request if the card is removed. Although we have already
check this in dw_mci_handle_cd for runtime usage of sd card and
dw_mci_init_slot for noremovable devices, but there is a timing
gap before it really calls dw_mci_get_cd as mmc_detect_change needs
some delay here.
Another gain here is that we could save some checkings of card status
after sd card been removed.