Add filter memory to frame.c for no passband change

https://github.com/Hamlib/Hamlib/issues/448

rigs/icom/frame.c

@@ -389,6 +389,7 @@ int rig2icom_mode(RIG *rig, vfo_t vfo, rmode_t mode, pbwidth_t width,
     unsigned char icmode;
     signed char icmode_ext;
     pbwidth_t width_tmp = width;
+    struct icom_priv_data *priv_data = (struct icom_priv_data *) rig->state.priv;
 
     icmode_ext = -1;
 
@@ -491,6 +492,11 @@ int rig2icom_mode(RIG *rig, vfo_t vfo, rmode_t mode, pbwidth_t width,
             }
         }
     }
+    else
+    {
+        // filter should already be set elsewhere
+        icmode_ext = priv_data->filter;
+    }
 
     *md = icmode;
     *pd = icmode_ext;