Fix checksum computation.  Previously it didn't include carry.

Reviewed by:	tuexen
Disable TX IP/TCP/UDP checksum offloading for RTL8168C/RTL8168CP.
Previously only TX IP checksum offloading was disabled but it's
reported that TX checksum offloading for UDP datagrams with IP
options also generates corrupted frames.  Reporter's controller is
RTL8168CP but I guess RTL8168C also have the same issue since it
shall share the same core.

Reported and tested by:	tuexen