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Higher speeds are only supported when PLL is enabled, but
the current driver doesn't enable PLL outside of stream
use cases, so better to set the lowest SPI speed accepted
by the entire device.

Move the current frequency set to the spi sub-driver so
the whole device can benefit from that speed.

spi-max-frequency property could be used, but ACPI systems don't
support it, so by setting it in the spi sub-driver probe
both Device Trees and ACPI systems are supported.
Signed-off-by: Lucas Tanure <tanureal@opensource.cirrus.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20211123163149.1530535-2-tanureal@opensource.cirrus.com

Signed-off-by: Mark Brown <broonie@kernel.org>