powerpc/pseries: Fix a typo in pSeries_lpar_hpte_insert()

Commit 801eb73f introduced
a bug while checking PTE flags. We have to drop the _PAGE_COHERENT flag
when __PAGE_NO_CACHE is set and the cache update policy is not write-through
(i.e. _PAGE_WRITETHRU is not set)

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
CC:  Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>