powerpc/vas: Export chip_to_vas_id()

Export the symbol chip_to_vas_id() to fix a build failure when
CONFIG_CRYPTO_DEV_NX_COMPRESS_POWERNV=m.

Fixes: d4ef61b5 ("powerpc/vas, nx-842: Define and use chip_to_vas_id()")
Reported-by: Haren Myneni <hbabu@us.ibm.com>
Reported-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>