power_control: generalize and rename lchan_ul_pf_ewma()
power_control: generalize and rename lchan_ul_pf_ewma()

This way EWMA based filtering logic can be used not only for
MS Power Control, but also for BS Power Control.

Change-Id: I16c2e1b997f2b8af44d47809420293f072335bbd
Related: SYS#4918

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