hwrng: bcm63xx - use devm_* helpers

Simplify the driver's probe function and error handling by using the
device managed allocators, while at it, drop the redundant "out of
memory" messages since these are already printed by the allocator.

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