crypto: dh - Make public key test FIPS-only

The function dh_is_pubkey_valid was added to for FIPS but it was
only partially conditional to fips_enabled.

In particular, the first test in the function relies on the last
test to work properly, but the last test is only run in FIPS mode.

Fix this inconsistency by making the whole function conditional
on fips_enabled.

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