mmc: Flush block queue when removing card

After mmc_block's remove function has exited, we must not
touch the card structure in any way. This means we not only
must remove the gendisk, we must also flush out any
remaning requests already queued up.

We previously removed the disk, but didn't flush it,
causing oops:es when removing a card in the middle of a
transfer.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>