mmc: dw_mmc: check card present before starting request

mmc: dw_mmc: check card present before starting request

The main reason to add this check is to avoid unnecessary
mmc_request like the on-going cmd and the corresponding sbc
if the card is removed. Although we have already checked 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.

Change-Id: Iea741c1c72985fbe078f48da3796bddcab816e66
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>