USB: switch ehci-hcd to new polling scheme

Switch ehci-hcd to use the new polling scheme, which reports root
hub status changes via the interrupt handler, in an asynchronous
fashion. Doing so disables polling for status changes (whose handler is
rh_timer_func).

Tested on a Geode GX machine, which is now capable of running at =~ 5
timer interrupts per second (in the -rt tree), resulting in significant
power savings.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>