rtl8169: Fixes, refactoring

- Fix spurious High Priority TX polling due to incorrect register definition
- Report a 1Gbps link instead of 10Gbps, this is purely for display purposes but Linux complains that 10Gbps LED doesn't exist
- Report RX descriptor protocol as TCP/IP to skip RX checksum in the driver
- Optimize rtl8169_interrupt() and other usages of atomic loads/stores
- Misc refactoring