sh: pci: Remove unused variable in SH-7786 PCI Express code

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

arch/sh/drivers/pci/pcie-sh7786.c

@@ -31,7 +31,6 @@ struct sh7786_pcie_port {
 
 static struct sh7786_pcie_port *sh7786_pcie_ports;
 static unsigned int nr_ports;
-static unsigned long dma_pfn_offset;
 size_t memsize;
 u64 memstart;