powerpc/mm: Fix fallthrough bug in hpte_decode

We should not fallthrough different case statements in hpte_decode. Add
break statement to break out of the switch. The regression is introduced by
dcda287a "powerpc/mm: Simplify hpte_decode"

Reported-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>