musb_hdrc: DaVinci-specific updates and cleanups

 - Add header needed for DaVinci build.
 - Remove some partially obsolete comments.
 - Cleaner and more reproducible host side VBUS handling, including
   waiting for it to fall after powerdown on rmmod.
 - Tighten up various state machine transitions and diagnostics

Plus removing some generic #ifdeffery.  The vbus updates make it work more like
the TUSB code, and the cleanups etc make upstream merging more practical.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>