qcacmn: Free serial cmd before its timer when rmmod

Rmmod process:
wlan_hdd_pld_remove --> hdd_stop_adapter --> hdd_vdev_destroy
--> WLAN_SER_CMD_DEL_STA_SESSION (25) cmd queued --> PLD_FW_DOWN
(will complete wait event as part of qdf_complete_wait_events)
--> hdd_wlan_stop_modules --> cds_disable -->
dispatcher_psoc_disable --> wlan_serialization_psoc_disable -->
wlan_serialization_timer_destroy --> umac_stop --> csr_stop -->
wlan_ser_cancel_non_scan_cmd -> check if timer is present but th
 timer are already destroyed, so assert.

Move wlan_serialization_purge_cmd_list into converged API file to
be shared by MCC and WIN.

Change-Id: Iad557e4a05d682c257be0c39049c52950e5eb530
CRs-Fixed: 2451058