hwrng: exynos - Signedness bug in exynos_trng_do_read()

"val" needs to be signed for the error handling to work.

Fixes: 6cd225cc

 ("hwrng: exynos - add Samsung Exynos True RNG driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>