add frequency and center frequency setting for SDR modulator

include/linux/dvb/mod.h

@@ -25,6 +25,7 @@ struct dvb_mod_channel_params {
 #define MODULATOR_FREQUENCY	  3
 #define MODULATOR_MODULATION	  4   
 #define MODULATOR_SYMBOL_RATE	  5   /* Hz */
+#define MODULATOR_BASE_FREQUENCY  6
 #define MODULATOR_ATTENUATOR     32  
 #define MODULATOR_INPUT_BITRATE  33  /* Hz */
 #define MODULATOR_PCR_MODE       34  /* 1=pcr correction enabled */