crypto: testmgr - fix initialization of 'secret_size'

Actual data length of the 'secret' is not equal to the 'secret_size'.

Since the 'curve_id' has removed in the 'secret', the 'secret_size'
should subtract the length of the 'curve_id'.

Fixes: 6763f5ea ("crypto: ecdh - move curve_id of ECDH from ...")
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>