phy: ralink: mt7621-pci: add sentinel to quirks table

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

phy: ralink: mt7621-pci: add sentinel to quirks table

With mt7621 soc_dev_attr fixed to register the soc as a device,

kernel will experience an oops in soc_device_match_attr

This quirk test was introduced in the staging driver in

commit 9445ccb3714c ("staging: mt7621-pci-phy: add quirks for 'E2'

revision using 'soc_device_attribute'"). The staging driver was removed,

and later re-added in commit d87da32372a0 ("phy: ralink: Add PHY driver

for MT7621 PCIe PHY") for kernel 5.11