crypto: virtio - constify features[] and id_table[]

features[] and id_table[] are not modified and can be made const to
allow the compiler to put them in read-only memory.

Before:
   text    data     bss     dec     hex filename
  11534    2056     160   13750    35b6 drivers/crypto/virtio/virtio_crypto_core.o

After:
   text    data     bss     dec     hex filename
  11630    1992     128   13750    35b6 drivers/crypto/virtio/virtio_crypto_core.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>