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/ADCSensor.cpp b/src/ADCSensor.cpp
index 4aa8cc1..330de52 100644
--- a/src/ADCSensor.cpp
+++ b/src/ADCSensor.cpp
@@ -176,7 +176,7 @@
         (*bridgeGpio).set(0);
     }
     int fd = open(path.c_str(), O_RDONLY);
-    if (fd <= 0)
+    if (fd < 0)
     {
         return; // we're no longer valid
     }