hwrng: core - start and stop in-kernel rngd in separate function

Extract the start/stop logic for the in-kernel rngd thread to
a separate function.

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>