crypto: lib/mpi - delete unnecessary condition

We checked that "nlimbs" is non-zero in the outside if statement so delete
the duplicate check here.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>