MFC r360467, r362399, r362405, r362415

r360467 by mmel:
Fix style(9). Strip write only variables.
Not a functional change.

r362399 by mmel:
Use naming nomenclature used in DesignWare TRM.
Use naming nomenclature used in DesignWare TRM.
This driver was written by using Altera (now Intel) documentation for Arria
FPGA manual. Unfortunately this manual used very different (and in some cases
opposite naming) for registers and descriptor fields. Unfortunately,
this makes future expansion extremely hard.

Should not been functional change.

r362405 by mmel:
Finish renaming in if_dwc.
By using DWC TRM terminology, normal descriptor format should be named
extended and alternate descriptor format should be named normal.

Should not been functional change.

r362415 by mmel:
Improve if_dwc:

• refactorize packet receive path. Make sure that we don't leak mbufs and/or that we don't create holes in RX descriptor ring
• slightly simplify handling with TX descriptors