Set MTU to 1200 for CST because we reserve 80 bytes for VCN. am: db501eb217 am: 780e2cb944 am: cdd70f2248

Original change: https://android-review.googlesource.com/c/platform/packages/services/Iwlan/+/2233786

Change-Id: Iba0129b60125038b2a191ed148a0f700bdcd4a7a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>