powerpc/pci: Fix regression in powerpc MSI-X

Patch f598282f exposed a problem in
powerpc MSI-X functionality, making network interfaces such as ixgbe
and cxgb3 stop to work when MSI-X is enabled. RX interrupts were not
being generated.

The problem was caused because MSI irq was not being effectively
unmasked after device initialization.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>