crypto: hisilicon - allow smaller reads in debugfs

Originally this code rejected any read less than 256 bytes.  There
is no need for this artificial limit.  We should just use the normal
helper functions to read a string from the kernel.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>