hwrng: amd - remove redundant initialization of variable err

The variable err is being initialized with a value that is
never read and it is being updated later with a new value.  The
initialization is redundant and can be removed

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>