crypto: picoxcell - set [src|dst]_nents and nents as signed int

The unsigned int variables [src|dst]_nents and nents can be assigned
signed value (-EINVAL) from sg_nents_for_len().
Furthermore they are used only by dma_map_sg and dma_unmap_sg which wait
for an signed int, so they must be set as int.

Fixes: f051f95e ("crypto: picoxcell - check return value of sg_nents_for_len")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>