Ensure that uC input pin P1.2_CP_RTS is valid when the CP2103 is off.

We accomplish this by enabling an internal pull-down on P1.2 of the MSP430.
It would be better to enable a pull-up, but then we are sending a voltage to
the CP2103 when it is off, which may cause a latch-up problem.  The pull-down
isn't terrible, since it represents the condition whereby the USB host is ready
to receive data from the RWS.  When the USB is connected, this is likely to be
the most common condition.