gsm_data: fix wrong variable set in gsm_pchan2chan_nr()

I believe the actual intention was to reset the 'lchan_nr' variable,
and not the 'chan_nr'.  The 'lchan_nr' is used to compose the 'cbits':

  cbits = 0x04;
  cbits += lchan_nr;

If the value is 4, then the result is:

  cbits = 0x04 + 4 = 0x08

which corresponds to SDCCH8 (not SDCCH4), and is clearly wrong.

Change-Id: Ic9c7c2e46e24dab0b721221e9adcbbae2ca56d23
Fixes: ec1b5a0e9 "gsm_ts2chan_nr(): add assertions for lchan_nr"
Fixes: CID#336586
---
M src/osmo-bsc/gsm_data.c
1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/src/osmo-bsc/gsm_data.c b/src/osmo-bsc/gsm_data.c
index 580fa84..4d77e75 100644
--- a/src/osmo-bsc/gsm_data.c
+++ b/src/osmo-bsc/gsm_data.c
@@ -485,8 +485,8 @@
 		 * See osmo-bts-xxx/oml.c:opstart_compl().
 		 */
 		if (lchan_nr == CCCH_LCHAN)
-			chan_nr = 0;
-		else if (lchan_nr >= 4)
+			lchan_nr = 0;
+		else if (lchan_nr > 4)
 			return -EINVAL;
 		cbits = 0x04;
 		cbits += lchan_nr;

