PCI: pcie-xilinx-nwl: Fix mask value to disable MSIs

Compiling the xilinx-nwl driver with sparse checks result in the
following warning:

drivers/pci/host/pcie-xilinx-nwl.c:633:38: sparse: cast truncates bits
from constant value (ffffffff00000000 becomes 0)

Fix it by explicitly writing 0 to mask interrupts instead of relying
on a bogus cast applied to the mask bitwise complement.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Michal Simek <michal.simek@xilinx.com>