hwrng: bcm2835 - remove redundant null check

clk_prepare_enable() and clk_disable_unprepare() will check
NULL clock parameter, so It is not necessary to add additional checks.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>