l1sap: use rxlev_full when no DTX was used
l1sap: use rxlev_full when no DTX was used

At the moment only rxlev_sub is used to decide when the facch repetition
should be activated, regardless if DTX is used. Lets check if DTX is
actually used and use rxlev_full when DTX is not applied.

Change-Id: I01ba57cf661f0e41b8df209c905f8d135013e472
Related: SYS#5114

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