hwrng: fetch randomness only after device init

Commit d9e79726 "hwrng: add randomness to system from rng sources"
added a call to rng_get_data() from the hwrng_register() function.
However, some rng devices need initialization before data can be read
from them.

This commit makes the call to rng_get_data() depend on no init fn
pointer being registered by the device.  If an init function is
registered, this call is made after device init.

CC: Kees Cook <keescook@chromium.org>
CC: Jason Cooper <jason@lakedaemon.net>
CC: Herbert Xu <herbert@gondor.apana.org.au>
CC: <stable@vger.kernel.org> # For v3.15+
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>