PCI: mediatek: Fixup MSI enablement logic by enabling MSI before clocks

Commit 43e6409d ("PCI: mediatek: Add MSI support for MT2712 and
MT7622") added MSI support but enabled MSI in the wrong place, at a step
in the probe sequence where clocks were not still enabled.

Fix this issue by calling mtk_pcie_enable_msi() in mtk_pcie_startup_port_v2()
since clocks are enabled when mtk_pcie_startup_port_v2() is called.

To avoid forward declaration of mtk_pcie_enable_msi(), move the
mtk_pcie_startup_port_v2() function definition in the file.

Fixes: 43e6409d

 ("PCI: mediatek: Add MSI support for MT2712 and MT7622")
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[lorenzo.pieralisi@arm.com: squashed commit and adapted log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>