cnss2: Add code to do PCIe L2 suspend during shutdown

During mhi resume if mhi detects link down then it posts link
recover event where cnss platform driver goes for self recovery,
as part of recovery the shutdown & powerup sequence is done.
During shutdown the PCIe link is suspended, the link suspend
should be L2 suspend. But in current scenario, it goes for DRV
suspend as the drv_connected_last flag is still set because
resume is initiated by host driver to get qmi stats via
cnss_pci_resume_bus. So on power up the link does not come up
as it tries for DRV resume instead of L2 resume. To fix this,
during shutdown the link suspend should be L2 suspend, so
add code to always do PCIe L2 suspend during shutdown.

Change-Id: I748d2e243bf4491ec31bc42027514a73c3d945b4
Signed-off-by: Mohammed Siddiq <msiddiq@codeaurora.org>