direct-phy: fix handle_ph_ra_ind(): handle PH-RA.ind on PRACH SAPI

In [1] I restricted L1 SAPI of PH-RA.ind to PDTCH and PTCCH, and
this seems to have caused a regression reported in [2]:

  DL1IF ERROR sysmo_l1_if.c:251 Rx PH-RA.ind for unknown L1 SAPI PRACH

I assumed that PH-RA.ind belonging to a Control Acknowledgement
message (in format of 4 Access Bursts) would have PDTCH SAPI,
while apparently it's actually arriving on PRACH.

[1] I482d60a46b9d253dfe0b16140eac9fea6420b30c
[2] https://osmocom.org/issues/1526#note-39

Change-Id: Ib0a6da37de7a1db4cad2b96293b31b9f32e7d9eb
Related: OS#1526
diff --git a/src/osmo-bts-sysmo/sysmo_l1_if.c b/src/osmo-bts-sysmo/sysmo_l1_if.c
index 5e743db..7406bbf 100644
--- a/src/osmo-bts-sysmo/sysmo_l1_if.c
+++ b/src/osmo-bts-sysmo/sysmo_l1_if.c
@@ -239,6 +239,7 @@
 
 	switch (ra_ind->sapi) {
 	case GsmL1_Sapi_Pdtch:
+	case GsmL1_Sapi_Prach:
 		bts_update_tbf_ta("PH-RA", ra_ind->u32Fn, fl1h->trx_no, ra_ind->u8Tn,
 				  qta2ta(ra_ind->measParam.i16BurstTiming), true);
 		break;