Added support for MSP430G2553

Added set of #defines to differentiate between the originally targeted
MSP430FR5739 and the MSP430G2553 that I wanted to use (because it
currently comes with the TI MSP430 Launchpad). MSP430G2553 has some
different register names and no DMA support.
Removed line accidentally left in from testing

Accidentally left in a #define BIT0 from testing whether it caused
compiler warnings; removed.