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.

Signed-off-by: Hamad Kadmany <qca_hkadmany@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>