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

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

Change-Id: I542390ff34de65d9a1b98bea6b795609fa319dc5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>