MCP55 is the only NVIDIA controller that supports VLAN tag
insertion/stripping and it also supports TSO over VLAN. Implement
TSO over VLAN support for MCP55 controller.

While I'm here clean up SIOCSIFCAP ioctl handler. Since nfe(4)
sets ifp capabilities based on various hardware flags in device
attach, there is no need to check hardware flags again in
SIOCSIFCAP ioctl handler. Also fix a bug which toggled both TX and
RX checksum offloading even if user requested either TX or RX
checksum configuration change.

Tested by:	Rob Farmer ( rfarmer <> predatorlabs dot net )