cnss2: Deinit MHI after suspending PCIe link

Since there is no graceful handling in WLAN device side to silence
all activities (e.g. page fault) during shutdown, it is possible
that while MHI is freeing remote memory, device is still trying to
access it which will casue page corruption. To avoid such issues,
suspend PCIe link before deinit MHI so that these accesses can be
rejected from PCIe.

Change-Id: Iee9e4e5ae425ac1ded441d9ade345bea4f9a56e9
Signed-off-by: Yue Ma <yuem@codeaurora.org>