crypto: hisilicon/hpre - Fix a erroneous check after snprintf()

This error handling looks really strange.
Check if the string has been truncated instead.

Fixes: 02ab9946

 ("crypto: hisilicon - Fixed some tiny bugs of HPRE")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>