apps/bttester: Update L2CAP COC MTU to be lower than application MTU
apps/bttester: Update L2CAP COC MTU to be lower than application MTU

Some testcases require MPS < MTU to test reassembly.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble