[PATCH] powerpc: Fix some MPIC + HT APIC buglets

Do disable, not enable, the HT APIC IRQ in the function that is
supposed to.
Enable the MPIC IRQ before enabling the downstream APIC IRQ, avoids
potentially losing an interrupt.

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>