hwrng: fetch randomness only after device init

hwrng: fetch randomness only after device init

Commit d9e7972619334 "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 <[email protected]>
CC: Jason Cooper <[email protected]>
CC: Herbert Xu <[email protected]>
CC: <[email protected]> # For v3.15+
Signed-off-by: Amit Shah <[email protected]>
Reviewed-by: Jason Cooper <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>