crypto: starfive - Fix nent assignment in rsa dec

Missing src scatterlist nent assignment in rsa decrypt function.
Removing all unneeded assignment and use nents value from req->src
instead.

Signed-off-by: Jia Jie Ho <jiajie.ho@starfivetech.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>