[PATCH] powerpc: Fix loss of interrupts with MPIC

With the new interrupt rework, an interrupt "host" map() callback can be
called after the interrupt is already active.

It's called again for an already mapped interrupt to allow changing the
trigger setup, and currently this is not guarded with a test of wether
the interrupt is requested or not.

I plan to change some of this logic to be a bit less lenient against
random reconfiguring of live interrupts but just not yet.

The ported MPIC driver has a bug where when that happens, it will mask
the interrupt.  This changes it to preserve the previous masking of the
interrupt instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>