mmc: atmel-mci: fix oops in atmci_tasklet_func

In some cases, a NULL pointer dereference happens because data is NULL when
STATE_END_REQUEST case is reached in atmci_tasklet_func.

Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Rodolfo Giometti <giometti@enneenne.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>