msp430: remove expensive modulo call
msp430: show overflow counter in hwtimer debug
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 and added timer debug.
test_hwtimer: hwtimer is automatically initialized now
msp430: don't enable Timer 0 by default
Merge pull request #909 from OlegHahm/msp430_hwtimer

msp430: hwtimer: handle overflow correctly
Fix a typo that provokes many unjustified "cc2420 unexpected IFG" alerts to occur