iio: adc: ti-ads1015: add 10% to conversion wait time

[ Upstream commit fe895ac8 ]

As user's guide "ADS1015EVM, ADS1115EVM, ADS1015EVM-PDK, ADS1115EVM-PDK
User Guide (Rev. B)" (http://www.ti.com/lit/ug/sbau157b/sbau157b.pdf)
states at page 16:
"Note that both the ADS1115 and ADS1015 have internal clocks with a ±10%
accuracy. If performing FFT tests, frequencies may appear to be incorrect
as a result of this tolerance range.", add those 10% to converion wait
time.

Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/adc/ti-ads1015.c

@@ -269,6 +269,7 @@ int ads1015_get_adc_result(struct ads1015_data *data, int chan, int *val)
 
 		conv_time = DIV_ROUND_UP(USEC_PER_SEC, data->data_rate[dr_old]);
 		conv_time += DIV_ROUND_UP(USEC_PER_SEC, data->data_rate[dr]);
+		conv_time += conv_time / 10; /* 10% internal clock inaccuracy */
 		usleep_range(conv_time, conv_time + 1);
 		data->conv_invalid = false;
 	}