PCI: mediatek: Fix possible truncation in mtk_pcie_parse_port()

PCI: mediatek: Fix possible truncation in mtk_pcie_parse_port()

As reported by the W=1 warning below, content of the 'name' variable might
get truncated with the existing size of 10 bytes. Though it is not
practically possible to exceed the 10 bytes size, increase it to 20 to
silence the warning for a clean W=1 build:

  $ 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);
	|         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
[mani: commit log]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/b835e360b42c5e0994f9301a34dbdf140a8d3ef5.1772493898.git.ryder.lee@mediatek.com