BTS: fix length of frames generated by f_TC_chan_act_encr

Since trxcon now drops L2 frames of an incorrect length, some
BTS tests which send unpadded frames with a short payload
have started to fail.

Extend the payload size generated by f_TC_chan_act_encr()
to make these tests pass again:

TC_chan_act_a51
TC_chan_act_a52
TC_chan_act_a53

Found by: Vadim Yanitskiy
Related: OS#3415

Change-Id: I0f9a40503a4ed4fce10d9655f845ac49d33f4041
1 file changed