MUSB: TUSB PM cleanup

Start updating the TUSB power related code ... first pass, use the chip's
idle mode whenever the USB link is inactive (disconnected or suspended).
That rule applies to both host and peripheral roles, and saves a fair
amount of power (50mA at 3.3V, etc).

Plus some cleanups.

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