target-ppc: Simplify HPTE matching

ppc_hash64_pteg_search() explicitly checks each HPTE's VALID and
SECONDARY bits, then uses the HPTE64_V_COMPARE() macro to check the B field
and AVPN.  However, a small tweak to HPTE64_V_COMPARE() means we can check
all of these bits at once with a suitable ptem value.  So, consolidate all
the comparisons for simplicity.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
