mailbox: bcm-pdc: Don't use iowrite32 to write DMA descriptors

In PDC driver, it is not necessary to use iowrite32()
when writing DMA descriptors to the transmit and receive rings.
The ring memory is in host memory. So convert to normal
assignment statements.

Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>