fix off-by-one in ADC sample rate divisor

Author: timchinowsky

ports/raspberrypi/common-hal/analogbufio/BufferedIn.c

@@ -53,7 +53,7 @@ void common_hal_analogbufio_bufferedin_construct(analogbufio_bufferedin_obj_t *s
     // sample rate determines divisor, not zero.
 
     // sample_rate is forced to be >= 1 in shared-bindings
-    float clk_div = (float)ADC_CLOCK_INPUT / (float)sample_rate;
+    float clk_div = (float)ADC_CLOCK_INPUT / (float)sample_rate - 1;
     adc_set_clkdiv(clk_div);
 
     // Set up the DMA to start transferring data as soon as it appears in FIFO