Add a hook in the IMU code for the camera board communications

Change-Id: Ib667ba814ce1e3e376252bba3375bedcf98ff707
diff --git a/frc971/wpilib/ADIS16448.cc b/frc971/wpilib/ADIS16448.cc
index 6c4a6d1..1107602 100644
--- a/frc971/wpilib/ADIS16448.cc
+++ b/frc971/wpilib/ADIS16448.cc
@@ -122,6 +122,8 @@
     : spi_(new frc::SPI(port)), dio1_(dio1) {
   // 1MHz is the maximum supported for burst reads, but we
   // want to go slower to hopefully make it more reliable.
+  // Note that the roboRIO's minimum supported clock rate appears to be
+  // 0.781MHz, so that's what this actually does.
   spi_->SetClockRate(1e5);
   spi_->SetChipSelectActiveLow();
   spi_->SetClockActiveLow();
@@ -298,6 +300,8 @@
     if (!message.Send()) {
       LOG(WARNING, "sending queue message failed\n");
     }
+
+    spi_idle_callback_();
   }
 }