PCI: designware-ep: Move the function of getting MSI capability forward

Move the function of getting MSI capability to the front of init
function, because the init function of the EP platform driver will use
the return value by the function of getting MSI capability.

Link: https://lore.kernel.org/r/20200918080024.13639-4-Zhiqiang.Hou@nxp.com


Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>