Comm: Fixed ambiguous cast that breaks compile.

I tried to update apm_planner but I had issues compling from source.  This is how I fixed the error that is below:
```
src/comm/MAVLinkDecoder.cc: In member function ‘QPair<QString, QVariant> MAVLinkDecoder::emitFieldValue(mavlink_message_t*, int, quint64)’:
src/comm/MAVLinkDecoder.cc:599:27: error: conversion from ‘uint64_t {aka long unsigned int}’ to ‘const QVariant’ is ambiguous
             retval.second = n;
```
I am using Ubuntu 14.04 which is up to date.