devices: pit: silence unused code warnings

Some of the variants of the CommandCounter enum are not currently used;
add a directive to ignore dead code warnings for these variants, since
they are defined by the hardware/spec and may be used in the future.

BUG=None
TEST='cargo build' executes without warnings

Change-Id: I72b6cd24722de801ebfe63bb7419c4e972463082
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1454139
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Miriam Zimmerman <mutexlox@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>
diff --git a/devices/src/pit.rs b/devices/src/pit.rs
index d1dc959..4a1bbbc 100644
--- a/devices/src/pit.rs
+++ b/devices/src/pit.rs
@@ -41,6 +41,7 @@
 // six bits of the byte. However, if 0xc0 is specified, it indicates that the
 // command is a "Read-Back", which can latch count and/or status of the
 // counters selected in the lower bits. See Intel 8254 data sheet for details.
+#[allow(dead_code)]
 #[derive(Debug, Clone, Copy, PartialEq, enumn::N)]
 enum CommandCounter {
     CommandCounter0 = 0x00, // Select counter 0.