Fix file descriptor checking

'0' is also a valid file descriptor so this commit fixes incorrect
checks on fd after opening a file. Actually, fd 0, 1 and 2 will be
assigned to stdin, stdout and stderr respectively so this patch
doesn't fix any issue but it's for code consistency.

Tested: Detecting and reading sensors worked well.

Change-Id: I2626a0b92352257d1c294a61ca385f95568dae7d
Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
diff --git a/src/CPUSensor.cpp b/src/CPUSensor.cpp
index 1b04299..0ec4bf5 100644
--- a/src/CPUSensor.cpp
+++ b/src/CPUSensor.cpp
@@ -204,7 +204,7 @@
     responseStream.clear();
     inputDev.close();
     int fd = open(path.c_str(), O_RDONLY);
-    if (fd <= 0)
+    if (fd < 0)
     {
         return; // we're no longer valid
     }