cnss2: Avoid forcing firmware assert if MHI is starting to power off

After MHI is starting to power off, there is no need to assert firmware
as it may fail at MHI layer itself.

Change-Id: I32f77a7b8f9d2a28529d76c90cd86faffd2e158d
Signed-off-by: Yue Ma <yuem@codeaurora.org>