msp430 usci: fix DMA TX byte overwrite

The DMA transmit operation for USCI was a little naive, in that it was
not accounting for the possibility that UCxxTXBUF already has a byte to
be transmitted.  Wait for this byte to be transmitted before starting
the DMA operation.

Also note in the code that the DMA transfer operations, trigered by
UCxxTXIFG, are edge sensitive.