msp430 adc12: If undefined ADC12ENC, assign from ENC.

TI's headers are inconsistent and some cpu's that have the
ADC12 hardware don't define ADC12ENC.  Rather it uses the older
unspecific ENC define.

This fix defines ADC12ENC from ENC but only for those cpus that
explicitly have the ADC12 h/w module.

Fixes tp-freeforall/prod #23