In case the probe of the clock is deferred, we would assume it is
optional. This is wrong, so defer the probe of this driver until
the clock is available.

Fixes: 791af4f4 ("hwrng: bcm2835 - Manage an optional clock")
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>