Fix compiler warning in media/mtp.

BUG=27152673

Change-Id: I0a5a7f2005bd76acf4d09353305b66af3fd29e4b
diff --git a/media/mtp/MtpDataPacket.cpp b/media/mtp/MtpDataPacket.cpp
index d0ec2a6..0381edf 100644
--- a/media/mtp/MtpDataPacket.cpp
+++ b/media/mtp/MtpDataPacket.cpp
@@ -456,7 +456,7 @@
         // look at the length field to see if the data spans multiple packets
         uint32_t totalLength = MtpPacket::getUInt32(MTP_CONTAINER_LENGTH_OFFSET);
         allocate(totalLength);
-        while (totalLength > length) {
+        while (totalLength > static_cast<uint32_t>(length)) {
             request->buffer = mBuffer + length;
             request->buffer_length = totalLength - length;
             int ret = transfer(request);