mmc: Avoid re-using minor numbers before the original device is closed.

Move the code which marks the minor number as free to mmc_blk_put() so
that it happens on the final close() (or removal), instead of doing it
at removal even when the device is still logically open.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>