PCI: dwc: Take lock when ACKing an interrupt

Bizarrely, there is no lock taken in the irq_ack() helper. This
puts the ACK callback provided by a specific platform in a awkward
situation where there is no synchronization that would be expected
on other callback.

Introduce the required lock, giving some level of uniformity among
callbacks.

Fixes: 7c5925af ("PCI: dwc: Move MSI IRQs allocation to IRQ domains
hierarchical API")
Link: https://lore.kernel.org/linux-pci/20181113225734.8026-1-marc.zyngier@arm.com/


Tested-by: Niklas Cassel <niklas.cassel@linaro.org>
Tested-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Tested-by: Stanimir Varbanov <svarbanov@mm-sol.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org