W=1 pcie-mediatek.c snprintf warnings

[Bjorn Helgaas]

Just noticed the following errors in W=1 builds.  I doubt there's any
real way to actually overflow the buffer given constraints in the
code, but these are the only warnings in drivers/pci/, so I'd like to
get them fixed so W=1 is more useful:

  $ make W=1 drivers/pci/controller/pcie-mediatek.o
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    INSTALL libsubcmd_headers
    CC      drivers/pci/controller/pcie-mediatek.o
  drivers/pci/controller/pcie-mediatek.c: In function ‘mtk_pcie_parse_port’:
  drivers/pci/controller/pcie-mediatek.c:963:43: error: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 6 [-Werror=format-truncation=]
    963 |         snprintf(name, sizeof(name), "port%d", slot);
	|                                           ^~
  drivers/pci/controller/pcie-mediatek.c:963:38: note: directive argument in the range [0, 2147483647]
    963 |         snprintf(name, sizeof(name), "port%d", slot);
	|                                      ^~~~~~~~
  drivers/pci/controller/pcie-mediatek.c:963:9: note: ‘snprintf’ output between 6 and 15 bytes into a destination of size 10
    963 |         snprintf(name, sizeof(name), "port%d", slot);
	|         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~