Addresses the following warning when building sdk7786_defconfig:

arch/sh/drivers/pci/pcie-sh7786.c:34:22: warning: 'dma_pfn_offset' defined but not used [-Wunused-variable]
   34 | static unsigned long dma_pfn_offset;
      |                      ^~~~~~~~~~~~~~

Fixes: e0d07278 ("dma-mapping: introduce DMA range map, supplanting dma_pfn_offset")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230419070934.422997-1-glaubitz@physik.fu-berlin.de