hwrng: core - remove (un)register_miscdev()

These functions are redundant after commit 0daa7a0a

 ("hwrng: Avoid
manual device_create_file() calls").

Let's call misc_(de)register() directly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>