usb-ehci: drop EXECUTING checks.

The state machine doesn't stop in EXECUTING state any more when async
packets are in flight, so the checks are not needed any more and can
be dropped.

Also kick out the check for the frame timer.  As we don't stop & sleep
any more on async packets this is obsolete.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
