Update ft991.c

Correct NR level

rigs/yaesu/ft991.c

@@ -166,7 +166,7 @@ const struct rig_caps ft991_caps =
     .has_set_parm =       RIG_PARM_NONE,
     .level_gran = {
 #include "level_gran_yaesu.h"
-        [LVL_NR] = { .min = { .f = 0.1 }, .max = { .f = 0.15 }, .step = { .f = 1.0f/10.0f } },
+        [LVL_NR] = { .min = { .f = 0 }, .max = { .f = 1 }, .step = { .f = 1.0f/15.0f } },
     },
     .ctcss_list =         common_ctcss_list,
     .dcs_list =           common_dcs_list,