dtc: Change exit code for usage message

If dtc's command line arguments are invalid, it prints a usage message
and returns exit code 2.  That's the same exit code as for a failed
check, which is potentially confusing if running dtc from an automated
harness.  Therefore this patch changes the usage exit code to 3.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>