PowerNV: send MSI_EOI always after MSI unmask

MSI/MSI-x interrupts are edge-triggered. If an interrupt
arrives when IRQ line is masked, it will be lost and will
never recover. Perform MSI_EOI always after unmask to give
a chance for PHB/XICS to send an interrupt again if MSI/MSI-x
pending bit is set in MSI/MSI-x BAR space.

Submitted by: Wojciech Macek <wma@semihalf.org>
Obtained from: Semihalf
Sponsored by: IBM, QCM Technologies