wlan: Split processing of assoc request frame

Split processing of assoc request frame and initialization of
PE/SME data structrures. This is needed to defer the initialization
of PE/SME entries on any external status. SAE authentication status
from hostapd is one such external status.

Change-Id: I9d7ced050441920505f0e12ab6b3ea9c672fe822
CRs-Fixed: 2734533