cnss2: Post register driver work as unkillable event

Register driver work handler will only power up device, resume PCIe
link and start MHI to download firmware. It may only block there
for a timeout, so there is no need to post it as killable event.

Change-Id: Ib82050358dc8df4a8163ecb43702aa86ed34f23d
Signed-off-by: Yue Ma <yuem@codeaurora.org>