msp430: cc430: handle hwtimer interrupts correctly

TAIFG should count the overflows, instead of using the first CCR for this purpose.
Without this commit ARCH_MAXTIMERS returns the wrong number as
effectively one timer less can be used.

Additional fixed comment style.
msp430: add timer debug
test_hwtimer: hwtimer is automatically initialized now
msp430: don't enable Timer 0 by default