KEYS: asymmetric: Fix sign/verify on pkcs1pad without a hash

The new sign/verify code broke the case of pkcs1pad without a
hash algorithm.  Fix it by setting issig correctly for this case.

Fixes: 63ba4d67

 ("KEYS: asymmetric: Use new crypto interface without scatterlists")
Cc: stable@vger.kernel.org # v6.5
Reported-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>