hwrng: core - Use DEVICE_ATTR_<RW|RO> macro

Use DEVICE_ATTR_RW()/DEVICE_ATTR_RO() helper instead of
plain DEVICE_ATTR, which makes the code a bit shorter and
easier to read.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>