hwrng: core - explicit ordering of initcalls

hw-random device drivers depend on the hw-random core being
initialized. Make this ordering explicit, also for the case
these drivers are built-in. As the core itself depends on
misc_register() which is set up at subsys_initcall time,
advance the initialization of the core (only) to the
fs_initcall() level.

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>