avraven: Fix the error CLOCK_CONF_SECOND

Since the match value is seted 32 and the prescale value is 256.

F_CPU = 8000000Hz
AVR_CONF_TMR0_PRESCALE == 256
OCR0A = 32

F_CPU = AVR_CONF_TMR0_PRESCALE * CLOCK_CONF_SECOND * OCR0A
CLOCK_CONF_SECOND = F_CPU / AVR_CONF_TMR0_PRESCALE / OCR0A
		  = 8000000 / 256 / 32
		  = 976.56

Signed-off-by: Zhong Hongbo <bocui107@gmail.com>
avraven: Correct the value of OCR0A

Since the match value is seted 32 and the prescale value is 256.

F_CPU = 8000000Hz
AVR_CONF_TMR0_PRESCALE == 256
F_CPU = AVR_CONF_TMR0_PRESCALE * CLOCK_CONF_SECOND * OCR0A

OCR0A = 32:
CLOCK_CONF_SECOND = F_CPU / AVR_CONF_TMR0_PRESCALE / OCR0A
		  = 8000000 / 256 / 32
		  = 976.56

OCR0A = 31:
CLOCK_CONF_SECOND = F_CPU / AVR_CONF_TMR0_PRESCALE / OCR0A
		  = 8000000 / 256 / 31
		  = 1008.064

Signed-off-by: Zhong Hongbo <bocui107@gmail.com>
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