qcacmn: Fix mem leak while deleting pmksa

Due to commit : I83e8d4c0c8b3ad503aa5894ffdc4a14bc3aeec7a,
while processing set_del_pmk command driver checks pmk_len
to delete pmk. In case if new PMK gets added with 0 lengths,
pmk entry with pmk_len = 0 will never be deleted. It is only
overwritten without freeing due to incorrect logic set_del
pmk logic.

Fix is to modify set and del pmk logic to avoid mem leak.

Change-Id: Idff573d020940dd926d07e1ec4f146eaa1215686
CRs-Fixed: 2696207