libcamera: change shutterspeed units to microseconds

This is to avoid shutter speeds less than 1ms from underflowing
during the conversion to the denominator portion of 1/1000s.

Change-Id: Ib6df304c38dd22a23aa31bc731c95030a089f05e
diff --git a/libcamera/SecCamera.cpp b/libcamera/SecCamera.cpp
index 83d100e..6f14198 100755
--- a/libcamera/SecCamera.cpp
+++ b/libcamera/SecCamera.cpp
@@ -3392,7 +3392,8 @@
         shutterSpeed = 100;
     }
     mExifInfo.exposure_time.num = 1;
-    mExifInfo.exposure_time.den = 1000.0 / shutterSpeed;   /* ms -> sec */
+    // x us -> 1/x s */
+    mExifInfo.exposure_time.den = (uint32_t)(1000000 / shutterSpeed);
 
     //3 ISO Speed Rating
     int iso = fimc_v4l2_g_ctrl(m_cam_fd, V4L2_CID_CAMERA_GET_ISO);