cxgbe(4): Add native netmap support to the main interface.

This means that extra virtual interfaces (VIs) created with
hw.cxgbe.num_vis are no longer required to use netmap. Use this
tunable to enable native netmap support on the main interface:

hw.cxgbe.native_netmap="3"

There is no change in default behavior.

Suggested by: jch@
MFC after: 2 weeks
Sponsored by: Chelsio Communications