USB: core: Add hub_get() and hub_put() routines

Create hub_get() and hub_put() routines to encapsulate the kref_get()
and kref_put() calls in hub.c.  The new routines will be used by the
next patch in this series.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/604da420-ae8a-4a9e-91a4-2d511ff404fb@rowland.harvard.edu


Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>