[][kernel][common][hnat][Fix Coverity scan warning]

[Description]
Fix Coverity scan warning.
    - Fix sscanf arbitrarily string issue in hnat_whnat_write().
    - Fix sscanf arbitrarily string issue in hnat_sched_write().
    - Fix sscanf arbitrarily string issue in hnat_static_entry_write().
    - Fix null check issue in mtk_pong_hqos_handler().
    - Fix uncheck return issue in mtk_hqos_ptype_cb().

[Release-log]
N/A


Change-Id: Ib6f772779d8b56233c355d50af7bdf1d72aedb11
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6861279
4 files changed