mmc: atmel-mci: Check pdata for NULL before dereferencing it at DMA config

Using an at91sam9g20ek development board with DTS configuration may trigger
a kernel panic because of a NULL pointer dereference exception, while
configuring DMA. Let's fix this by adding a check for pdata before
dereferencing it.

Signed-off-by: Brent Taylor <motobud@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>