wil6210: protect list of pending wmi events during flush

When flush is done, pending events list is manipulated
without taking the proper spinlock, which could lead to
memory corruption if list is manipulated by wmi worker
or by interrupt routine.

Change-Id: Iccbc65d94f837402c12d9794cfdde21339599a0b
CRs-Fixed: 2002638
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>