spi: imx: mx51-ecspi: Reinstate low-speed CONFIGREG delay

spi: imx: mx51-ecspi: Reinstate low-speed CONFIGREG delay

Since 00b80ac935539 ("spi: imx: mx51-ecspi: Move some initialisation to
prepare_message hook."), the MX51_ECSPI_CONFIG write no longer happens
in prepare_transfer hook, but rather in prepare_message hook, however
the MX51_ECSPI_CONFIG delay is still left in prepare_transfer hook and
thus has no effect. This leads to low bus frequency operation problems
described in 6fd8b8503a0dc ("spi: spi-imx: Fix out-of-order CS/SCLK
operation at low speeds") again.

Move the MX51_ECSPI_CONFIG write delay into the prepare_message hook
as well, thus reinstating the low bus frequency fix.

Fixes: 00b80ac935539 ("spi: imx: mx51-ecspi: Move some initialisation to prepare_message hook.")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210703022300.296114-1-marex@denx.de
Signed-off-by: Mark Brown <broonie@kernel.org>