mt7621 static check warning

[Bjorn Helgaas]

Hi Sergio,

FYI:

  $ make W=1 drivers/pci/
    CC      drivers/pci/controller/pcie-mt7621.o
  drivers/pci/controller/pcie-mt7621.c: In function ‘mt7621_pcie_probe’:
  drivers/pci/controller/pcie-mt7621.c:228:49: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
    228 |         snprintf(name, sizeof(name), "pcie-phy%d", slot);
	|                                                 ^
  drivers/pci/controller/pcie-mt7621.c:228:9: note: ‘snprintf’ output between 10 and 11 bytes into a destination of size 10
    228 |         snprintf(name, sizeof(name), "pcie-phy%d", slot);
	|         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I know we'll never actually hit this, but it'd be nice to clean this
up, and I don't think it would really cost us anything.  I think it's
currently the only "W=1" warning in drivers/pci/.

Bjorn