mmc: dw_mmc: Stop bounce buffer even in case of failure

The driver didn't stop the bounce buffer in case a data transfer
failed. This would lead to memory leakage if the communication
between the CPU and the card is unreliable. Add the missing call
to stop the bounce buffer.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>