Cast size_t to DWORD to avoid compiler warning.

This doesn't change any functionality, but silences the following compiler warning (which occurs at least on Windows x64, not sure about elsewhere):

```
..\src\serialport_win.cpp(281): warning C4267: 'argument' : conversion from 'size_t' to 'DWORD', possible loss of data
[c:\Users\Domenic\Dropbox\Programming\WIP\nodebots\node_modules\serialport\build\serialport.vcxproj]
```