cnss2: Avoid blocking target to reboot or shutdown

Use wait_for_completion_killable() instead of wait_for_completion() to
avold blocking target to reboot or shutdown during WLAN device power
on/off.

Change-Id: If60848fe9d61e506347e3c6b0fe8c8604cee6c79
Signed-off-by: Yue Ma <yuem@codeaurora.org>