cnss2: Notify MHI driver immediately after PCIe linkdown

Due to serialization, PCIe linkdown notification to MHI driver may
be queued in the driver event workqueue. This results MHI driver
timeout first and thereafter handle the linkdown. Notify MHI driver
immediately after PCIe linkdown so it is able to handle as early as
possible.

Change-Id: I115aedf17b1b2381560bb4c12039223587da4cc4
Signed-off-by: Yue Ma <yuem@codeaurora.org>