grove.cxx: Improve GroveTemp formula

Signed-off-by: Sarah Knepper <sarah.knepper@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/grove/grove.cxx

@@ -81,9 +81,9 @@ GroveTemp::~GroveTemp()
 int GroveTemp::value ()
 {
     int a = mraa_aio_read(m_aio);
-    float r = (float)(1023-a)*10000/a;
-    float t = 1/(log(r/10000)/3975 + 1/298.15)-273.15;
-    return (int) t;
+    float r = (float)(1023.0-a)*10000.0/a;
+    float t = 1.0/(log(r/10000.0)/3975.0 + 1.0/298.15)-273.15;
+    return (int) round(t);
 }
 
 float GroveTemp::raw_value()