tty: amba-pl011: don't dereference NULL platform data

When only a TX DMA channel is specified in DT, pl011_dma_probe() falls
back to looking for the optional RX channel in platform data. What it
doesn't do is check whether that platform data actually exists...

Add the missing check to avoid crashing the kernel.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>