iio: adc: ad7380: fix oversampling formula

iio: adc: ad7380: fix oversampling formula

The formula in the datasheet for oversampling time conversion seems to
be valid when device is at full speed using the maximum number of SDO
lines. The driver currently support only 1 SDO line. The correct formula
is: t_convert = T_CONVERT_0_NS + T_CONVERT_X_NS*(x -
1)*num_channel/number_of_sdo_lines.
It will produce larger delays than what is currently set, but some devices
actually require it.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241030-ad7380-add-adaq4380-4-support-v4-2-864ff02babae@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>