crypto: public_key - Make sig/tfm local to if clause in software_key_query

The recent code changes in this function triggered a false-positive
maybe-uninitialized warning in software_key_query.  Rearrange the
code by moving the sig/tfm variables into the if clause where they
are actually used.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>