crypto: sun8i-ce - fix error return code in sun8i_ce_prng_generate()

When dma_mapping_error() returns an error, no error return code of
sun8i_ce_prng_generate() is assigned.
To fix this bug, err is assigned with -EFAULT as error return code.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>