USB: EHCI: add a name for the platform-private field

This patch (as1642) adds an ehci->priv field for private use by EHCI
platform drivers.  The space was provided some time ago, but it didn't
have a name.

Until now none of the platform drivers has used this private space,
but that's about to change in the next patch of this series.

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