Incorrect type in SPI_GetNumChannels()

[mgthompson]

numChannels should be UInt32, not int to match LibMPSSE.dll.

    [DllImport(LibMpsse.DllName, SetLastError = true, CallingConvention = CallingConvention.Cdecl)]
    public extern static FtResult SPI_GetNumChannels(out UInt32 numChannels);