powerpc/powernv/vas: Define vas_win_close() interface

powerpc/powernv/vas: Define vas_win_close() interface

BugLink: http://bugs.launchpad.net/bugs/1718293
Define the vas_win_close() interface which should be used to close a
send or receive windows.

While the hardware configurations required to open send and receive
windows differ, the configuration to close a window is the same for
both. So we use a single interface to close the window.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 98271d4198699947d66d6f8a02c09bd27cb90022)
Signed-off-by: Gustavo Walbon <gwalbon@linux.vnet.ibm.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>