crypto: ccree - use std api when possible

Move to use the std api sg_nents_for_len() when we do not in fact
require the extra information about the number of bytes in the last
entry provided by the in-driver variant cc_get_sgl_nents().

This also resolves a Coverity warning cause by us not using
the output value.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>