USB: change hub initialization sleeps to delayed_work

This patch (as1137) changes the hub_activate() routine, replacing the
power-power-up and debounce delays with delayed_work calls.  The idea
is that on systems where the USB stack is compiled into the kernel
rather than built as modules, these delays will no longer block the
boot thread.  At least 100 ms is saved for each root hub, which can
add up to a significant savings in total boot time.

Arjan van de Ven was very pleased to see that this shaved 700 ms off
his computer's boot time.  Since his total boot time is on the order
of two seconds, the improvement is considerable.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>