cnss2: Handle race between register driver and reboot properly

Register driver should be skipped if reboot is already in progress.
Also firmware boot timer should be stopped if register driver work
has been scheduled but gets killed later to avoid timing out because
of the reboot.

Change-Id: Iae4de860329a82416bfa58967095a4de7b509813
Signed-off-by: Yue Ma <yuem@codeaurora.org>