hwrng: atmel - Use devm_clk_get()

Use devm_clk_get() to make cleanup paths simpler.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>