mxge: Clean up RX path
mxge: inline -> __inline
mxge: Cleanup TX descriptor capsulation
mxge: Remove several unused fields in softc
mxge: Fix if_start implementation

- Don't transmit, if interface is not up or OACTIVE
- Merge mxge_start_locked into mxge_start; we won't need locked version
  of if_start
mxge: Cleanup TX completion path

While im here, remove unused stats fields in softc