Garmin: Fix gas change event parsing

The gas mixes are numbered from 0 and up and the gas change event uses
this number directly, so no need to subtract one.

Signed-off-by: Michael Andreen <michael@andreen.dev>

src/garmin_parser.c

@@ -217,7 +217,7 @@ static void garmin_event(struct garmin_parser_t *garmin,
 		return;
 
 	case 57:
-		sample.gasmix = data - 1;
+		sample.gasmix = data;
 		garmin->callback(DC_SAMPLE_GASMIX, sample, garmin->userdata);
 		return;
 	}