USB: update last_busy field correctly

This patch (as966) fixes a bug in the autosuspend code.  The last_busy
field should be updated whenever any event occurs, not just events
that cause an autosuspend or an autoresume.

This partially fixes Bugzilla #8892.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>