power_control: derive calc_delta() from lchan_ms_pwr_ctrl()
power_control: derive calc_delta() from lchan_ms_pwr_ctrl()

This function will also be used by the BS Power Control logic.

Change-Id: I2525eb2c3f1fdb3d0847a8ad50b30a44d7741a55
Related: SYS#4918

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