USB: ehci: fix debugfs 'lpm' permissions

The permissions for the lpm debugfs file is incorrect, this fixes it.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alek Du <alek.du@intel.com>
Cc: Jacob Pan <jacob.jun.pan@intel.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>