cnss2: Add mutex protection between unregister driver and idle restart

Since idle restart is a multiple stages power up sequence, there will be
race conditions when idle restart and unregister driver (which is power
down sequence) come at the same time. Add mutex protection for them so
that they can be serialized completely.

Change-Id: Icc28b5f759cd690cb505b16ab70eaa4abaf2e4cc
Signed-off-by: Yue Ma <yuem@codeaurora.org>