PM / Runtime: Automatically retry failed autosuspends

Originally, the runtime PM core would send an idle notification
whenever a suspend attempt failed.  The idle callback routine could
then schedule a delayed suspend for some time later.

However this behavior was changed by commit
f71648d7

 (PM / Runtime: Remove idle
notification after failing suspend).  No notifications were sent, and
there was no clear mechanism to retry failed suspends.

This caused problems for the usbhid driver, because it fails
autosuspend attempts as long as a key is being held down.  Therefore
this patch (as1492) adds a mechanism for retrying failed
autosuspends.  If the callback routine updates the last_busy field so
that the next autosuspend expiration time is in the future, the
autosuspend will automatically be rescheduled.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Henrik Rydberg <rydberg@euromail.se>
Cc: <stable@kernel.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>