parisc: pci-dma: fix warning unused-function

When building tinyconfig on parisc the following warnign shows up:

/tmp/arch/parisc/kernel/pci-dma.c:338:12: warning: 'proc_pcxl_dma_show' defined but not used [-Wunused-function]
 static int proc_pcxl_dma_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~~~~~~

Mark the function as __maybe_unused to fix the warning.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>