hwrng: core - only set cur_rng_set_by_user if it is working

In case the user-specified rng device is not working, it is not used;
therefore cur_rng_set_by_user must not be set to 1.

Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>