msm: ipa: fix security issues in ipa wan driver

Fix the security issue in handling add mux channel event
in ipa wan driver.

Bug: 36490777
Change-Id: Ic2ffeafddad4954ec3ecba0d675646d0790eede7
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>
Acked-by: Shihuan Liu <shihuanl@qti.qualcomm.com>