PCI: mt7621: Add sentinel to quirks table

In the Linux kernel, the following vulnerability has been resolved:

PCI: mt7621: Add sentinel to quirks table

Current driver is missing a sentinel in the struct soc_device_attribute

array, which causes an oops when assessed by the

soc_device_match(mt7621_pcie_quirks_match) call.

This was only exposed once the CONFIG_SOC_MT7621 mt7621 soc_dev_attr

was fixed to register the SOC as a device, in:

commit 7c18b64bba3b ("mips: ralink: mt7621: do not use kzalloc too early")

Fix it by adding the required sentinel.