This is a general cleanup of em_txrx.
- Correct the E1000_TXD_CMD_IP bit (8254x SDM4.0 page 45, and PCIe GbE SDM2.5 page 63)
- Move the documentation about csum offload contexts and the DONT_FORCE_CTX define inline so it explains what is going on closer to the subject
- Move HWCSUM disables out of txrx to setup (< e1000_82543 already disabled), new if statement handles 82547 limitations
- Correct lem-class VLAN tagging (need E1000_RXD_SPC_VLAN_MASK)
- Add CSUM_IPV6 and TSO6 for em-class. TSO is disabled by default but this should now work.