_mmc_detect_card_removed() validates that the card is removable, but when
being called via the bus_ops ->detect() callbacks, the validation is
redundant as it's already done in mmc_rescan().

Move the validation of a removable card to the mmc_detect_card_removed()
API, which is where it's applicable, to allow the blk error recovery path
to get the response a bit earlier.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>