Detect full 48 bits of last data frame in a transmission

While testing new support for DV Fast Data, I ran across a particular
image that reliably generated a bit sequence that the MDMVM firmware
interpreted as an end-of-transmission.  I dug a bit and discovered
that MMDVM only matches on 32 bits of the last data frame instead of
the full 48 bits.

See http://www.arrl.org/files/file/D-STAR.pdf section 2.1.2, item (6)
for details.

DStarRX.h

@@ -41,6 +41,7 @@ private:
   bool         m_prev;
   DSRX_STATE   m_rxState;
   uint32_t     m_patternBuffer;
+  uint64_t     m_patternBuffer64;
   uint8_t      m_rxBuffer[100U];
   unsigned int m_rxBufferBits;
   unsigned int m_dataBits;