libnfc-nci fixes for AArch64 build.

pthread_cond_timedwait_monotonic() is going away in LP64.
Use the POSIX standard for now, and use pthread_condattr_setclock()
when it becomes available to set clock spec to MONOTONIC.

Also fixed some other small build warnings/errors.

Change-Id: Ic192299b157529fa3f74dba521f038b8b99206bf
diff --git a/src/adaptation/NfcAdaptation.cpp b/src/adaptation/NfcAdaptation.cpp
index 3eb55ea..c30b97b 100755
--- a/src/adaptation/NfcAdaptation.cpp
+++ b/src/adaptation/NfcAdaptation.cpp
@@ -212,7 +212,7 @@
     ALOGD ("%s: enter", func);
     GKI_run (0);
     ALOGD ("%s: exit", func);
-    return NULL;
+    return 0;
 }
 
 /*******************************************************************************
@@ -240,7 +240,7 @@
 
     GKI_exit_task (GKI_get_taskid ());
     ALOGD ("%s: exit", func);
-    return NULL;
+    return 0;
 }
 
 /*******************************************************************************