qcacmn: enable legacy coex feature

Because newest firmware codes already been changed, the previous
coex configure command are invalid. Firmware need host driver
to enable WMI_SERVICE_THREE_WAY_COEX_CONFIG_LEGACY service during
wlan init. Add a new parameter "gThreeWayCoexConfigLegacyEnable" to
initial config file. If this parameter value is zero,set coex
priorities command will return error.

Change-Id: Ief1617e2ba5363e276c55135198613a0e54b71d5
CRs-Fixed: 2482742