tbf: Fix copy and paste in the set_mode routine

This is the second attempt to fix what looks like a copy and paste
issue. The code assigns m_current_cs_ul and then compares the _dl
variant, then assigns m_current_cs_ul with a default value. It seems
to indicate that _ul should be used.

Fixes: Coverity: CID 1351733
diff --git a/src/gprs_ms.cpp b/src/gprs_ms.cpp
index 78f03f8..4296fd3 100644
--- a/src/gprs_ms.cpp
+++ b/src/gprs_ms.cpp
@@ -237,7 +237,7 @@
 		if (!m_current_cs_ul.isEgprs()) {
 			m_current_cs_ul = GprsCodingScheme::getEgprsByNum(
 				m_bts->bts_data()->initial_mcs_ul);
-			if (!m_current_cs_dl.isValid())
+			if (!m_current_cs_ul.isValid())
 				m_current_cs_ul = GprsCodingScheme::MCS1;
 		}
 		if (!m_current_cs_dl.isEgprs()) {