hwrng: bcm2835 - Handle deferred clock properly

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>