This patch starts adding trace support to ehci.  It traces
updates of the status register (USBSTS), mmio access and
controller reset.

It also adds functions to set and clear status register bits
and puts them in use everywhere.

Some DPRINTF's are dropped in favor of the new tracepoints.

No change in behavior.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>