e1000: Some fixes for em(4) TSO setup
Always set TXD_CMD_IP for 82544
Otherwise set TXD_CMD_IP for IPv4, not IPv6
Reviewed by: markj (previous version)
Differential Revision: https://reviews.freebsd.org/D30072
(cherry picked from commit 201c4b7c29da44500ccb9e47b854dc1207df8b0a)