powerpc/pseries: Return req#msi(-x) if request is larger

If a driver asks for more MSIs than the devices "req#msi(-x)" property,
we currently return -ENOSPC. This doesn't give the driver any chance to
make a new request with a number that might work.

So if "req#msi(-x)" is less than the request, return its value. To be
100% safe, make sure we return an error if req_msi == 0.

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