Remove hard-coded rx execution context of wmi event callbacks.
UMAC layer needs to register execution context while registering
events with wmi layer.
Execute wmi rx event callbacks in following context-
1) work queue context provided by wmi layer.
2) umac provided context, umac can choose to
execute in thread context or default tasklet context
based on umac implementation.
CRs-Fixed: 978619
Change-Id: Id9b1f1e5157e8a203f93a62791f85a8145cc1599
