crypto: nx - Remove the unneeded result variable

Return the value set_msg_len() directly instead of storing it in another
redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>