powerpc/pseries: Return the number of MSIs we could allocate

If we can't allocate the requested number of MSIs, we can still tell the
generic code how many we were able to allocate. That can then be passed
onto the driver, allowing it to request that many in future, and
probably succeeed.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>