+ *
+ * Because spi_imx->spi_bus_clk is only set in bitbang prepare_message
+ * callback, iterate over all the transfers in spi_message, find the
+ * one with lowest bus frequency, and use that bus frequency for the
+ * delay calculation. In case all transfers have speed_hz == 0, then
+ * min_speed_hz is ~0 and the resulting delay is zero.