PCI: cadence: Write MSI data with 32bits

According to the PCIe specification, although the MSI data is only
16bits, the upper 16bits should be written as 0. Use writel
instead of writew when writing the MSI data to the host.

Fixes: 37dddf14 ("PCI: cadence: Add EndPoint Controller driver for Cadence PCIe controller")
Signed-off-by: Alan Douglas <adouglas@cadence.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>