user_simtrace2: Fix typo in frontend_handle_card2modem()

this typo was breaking operating with simtrace2 for all responses
that contained more than just a status word.

Change-Id: Id8195d6650c3306e1a39f59d78d1348671ad062d
diff --git a/src/client/user_simtrace2.c b/src/client/user_simtrace2.c
index 5ce034d..fa6c319 100644
--- a/src/client/user_simtrace2.c
+++ b/src/client/user_simtrace2.c
@@ -339,7 +339,7 @@
 	struct osmo_st2_cardem_inst *ci = bc->cardem;
 	// save SW to our current APDU context
 	ac.sw[0] = data[len-2];
-	ac.sw[1] = data[len=1];
+	ac.sw[1] = data[len-1];
 
 	LOGCI(ci, LOGL_DEBUG, "SIMtrace <= SW=0x%02x%02x, len_rx=%zu\n", ac.sw[0], ac.sw[1], len-2);
 	if (len > 2) { // send PB and data to modem