hwrng: virtio - Constify id_table[]

id_table[] is not modified and an be made const to allow the compiler to
put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
   1746     192       8    1946     79a drivers/char/hw_random/virtio-rng.o

After:
   text    data     bss     dec     hex filename
   1762     176       8    1946     79a drivers/char/hw_random/virtio-rng.o

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