MUSB: Update register access based on new HDRC core spec

Catch up to info from latest HDRC core spec ... some new registers
and fields, and constraints notably

 - not clearing TX fifo unless it's empty (that might explain some
   host side glitching).

 - not all POWER fields are writable

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

diff --cc drivers/usb/musb/g_ep0.c
Simple merge

diff --cc drivers/usb/musb/musb_gadget.c
Simple merge

diff --cc drivers/usb/musb/musb_host.c
Simple merge

diff --cc drivers/usb/musb/musbhdrc.h
Simple merge

diff --cc drivers/usb/musb/plat_uds.c
Simple merge

diff --cc drivers/usb/musb/virthub.c
Simple merge