measurement: count all blocks as SUB for TCH/F in signalling mode
measurement: count all blocks as SUB for TCH/F in signalling mode

SUB frames exist only in voice (or CSD) channels. When a TCH/F is in
signalling mode, all blocks must be counted as SUB blocks. (for TCH/H
the current implementation is correct.)

Change-Id: I04be21200afa1d03afa0d7e476c66fa79cf42249
Related: OS#4799

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts