mt76: mt7603: add additional EEPROM chip ID (#2133)

Some newer MT7628 based routers (notably the TP-Link Archer C50 v4) are
shipped with a chip-id of 0x7600 in the on-flash EEPROM. Add this as a
possible valid ID.

This fixes unstable WiFi on some units of the TP-Link Archer C50 v4.