Update drv_hrt.c

TIM3 was not working properly with a custom application and I got it work changing TIM3 HRT_TIMER_POWER_BIT from RCC_APB2ENR_TIM3EN to RCC_APB1ENR_TIM3EN because on datasheet TIM3 is on APB1, I think that you should check also the others
Merge pull request #839 from eneadev/patch-1

Update drv_hrt.c