USB: EHCI: Export the ehci_hub_control function

Platform drivers sometimes need to perform specific handling of hub
control requests. Make this possible by exporting the ehci_hub_control()
function which can then be called from a custom hub control handler in
the default case.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>