Update drv_hwtimer.c
Update drv_hwtimer.c

STM32的硬件定时器在 APBxCLKDivider 为 RCC_HCLK_DIV1 的时候，timer clocks 频率等于外设总线时钟PCLKx. 反之 timer clocks 为 PCLKx 的2倍.
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