Fix a few things in one shot.
- clean up some code to avoid potential NULL pointer dereference.
- add a helper function - wlan_ipa_free_tx_desc_list() to
avoid duplication.
- rearrage wlan_ipa_teardown_sys_pipe() to allow others' reference
Change-Id: I9d6391a7f20ae427bf59f07958bd13f349e61d83
CRs-Fixed: 2233867