pcu_l1_if_phy: add new PHY API function to disconnect PDCH

There is a function l1if_connect_pdch, but no complementary function
like we have it with l1if_open_pdch and l1if_close_pdch. The reason for
this is that the PHY implementations that rely on a femtocell DSP do not
need to disconnect the pdch explcitly. However, the planned support for
the E1 based Ercisson RBS CCU will require an explicit disconnect. So
lets add a function call for this.

Change-Id: Ied88f3289bda87c48f5f9255c4591470633cc805
Related: OS#5198
diff --git a/src/osmo-bts-sysmo/sysmo_l1_if.c b/src/osmo-bts-sysmo/sysmo_l1_if.c
index 31028f5..18378b6 100644
--- a/src/osmo-bts-sysmo/sysmo_l1_if.c
+++ b/src/osmo-bts-sysmo/sysmo_l1_if.c
@@ -128,6 +128,11 @@
 	return l1if_req_pdch(fl1h, msg);
 }
 
+int l1if_disconnect_pdch(void *obj, uint8_t ts)
+{
+	return 0;
+}
+
 static int handle_ph_readytosend_ind(struct femtol1_hdl *fl1h,
 				     GsmL1_PhReadyToSendInd_t *rts_ind)
 {