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>

drivers/char/hw_random/core.c

@@ -335,8 +335,9 @@ static ssize_t rng_current_store(struct device *dev,
 	} else {
 		list_for_each_entry(rng, &rng_list, list) {
 			if (sysfs_streq(rng->name, buf)) {
-				cur_rng_set_by_user = 1;
 				err = set_current_rng(rng);
+				if (!err)
+					cur_rng_set_by_user = 1;
 				break;
 			}
 		}