hwrng: via_rng - Fix asm constraints

The inline asm to invoke xstore did not specify the constraints
correctly.  In particular, dx/di should have been marked as output
registers as well as input as they're modified by xstore.

Thanks to Mario Holbe for creating this patch and testing it.

Tested-by: Mario 'BitKoenig' Holbe <Mario.Holbe@TU-Ilmenau.DE>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>