timer loop in adc.c

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Another one for when I've finished playing with mspider's changes:

adc calibration timers may not wait as expected if startup times are long:

ui16_counter = TIM3_GetCounter() + 6000;
while(TIM3_GetCounter() < ui16_counter) ;

works ok if initial timer values are < (65535-6000) - values above that the loop won't wait at all...