l1sap: Split ph_data_req() into smaller parts
l1sap: Split ph_data_req() into smaller parts

... in an effort to avoid introducing new/more spaghetti code

Also, use offsetof() instead of pointer calculation to determine
the start of GsmL1_Prim_t.u.phDataReq.msgUnitParam.u8Buffer

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