Update FT991 caps with agc levels

#886

rigs/yaesu/ft991.c

@@ -178,6 +178,8 @@ const struct rig_caps ft991_caps =
     .max_rit =            Hz(9999),
     .max_xit =            Hz(9999),
     .max_ifshift =        Hz(1200),
+    .agc_level_count = 5,
+    .agc_levels = { RIG_AGC_OFF, RIG_AGC_FAST, RIG_AGC_MEDIUM, RIG_AGC_SLOW, RIG_AGC_AUTO },
     .vfo_ops =            FT991_VFO_OPS,
     .targetable_vfo =     RIG_TARGETABLE_FREQ,
     .transceive =         RIG_TRN_OFF,        /* May enable later as the 950 has an Auto Info command */