gsm_data: return early if MS Power class remains the same

The following message makes no sense:

  DRLL DEBUG gsm_data.c:844 MS Power class update: 4 -> 4

because nothing really changed, MS Power class remains 4.  Neither
it makes sense to call lchan_update_ms_power_ctrl_level().

Change-Id: I519d2d1575cbb5352cc381a60513db8e0e2cb0a0
1 file changed