msm: pcie: add proper PCIe link state for linkdown

When PCIe root complex receives a linkdown, move PCIe S/W state
to MSM_PCIE_LINK_DOWN instead. When using MSM_PCIE_LINK_DISABLED,
there is a chance PCIe resume will execute even though PCIe core
and PHY is still up.

Change-Id: Ifee3c60d5eedf1fd73751fa13905e9f32d17bbae
Signed-off-by: Tony Truong <truong@codeaurora.org>