xtest_20000.c: Fix fopen() mode string

The correct fopen() mode string to open an existing file in read/write
mode and set the stream pointer at the beginning is 'r+'.
'rw+' is invalid and causes the file to be open read-only on Android.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
diff --git a/host/xtest/xtest_20000.c b/host/xtest/xtest_20000.c
index 9a265e3..9e6c2f0 100644
--- a/host/xtest/xtest_20000.c
+++ b/host/xtest/xtest_20000.c
@@ -349,7 +349,7 @@
 			offset = rand() % (st.st_size-1);
 		}
 
-		fd = fopen(name, "rw+");
+		fd = fopen(name, "r+");
 		if (!fd)
 			goto exit;