powerpc/vas, nx-842: Define and use chip_to_vas_id()

Define a helper, chip_to_vas_id() to map a given chip id to corresponding
vas id.

Normally, callers of vas_rx_win_open() and vas_tx_win_open() want the VAS
window to be on the same chip where the calling thread is executing. These
callers can pass in -1 for the VAS id.

This interface will be useful if a thread running on one chip wants to open
a window on another chip (like the NX-842 driver does during start up).

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>