msm: rmnet_ipa: fix security issue

Fix the security issue where mux channel name might
not be null-terminated in ipa wan driver.

Change-Id: I3ef440b62cf3861464fb60c1e7f65f2be5e39ed0
Acked-by: Shihuan Liu <shihuanl@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>