media: imx-mipi-csis: Only set clock rate when specified in DT

The imx-mipi-csis driver sets the rate of the wrap clock to the value
specified in the device tree's "clock-frequency" property, and defaults
to 166 MHz otherwise. This is a historical mistake, as clock rate
selection should have been left to the assigned-clock-rates property.

Honouring the clock-frequency property can't be removed without breaking
backwards compatibility, and the corresponding code isn't very
intrusive. The 166 MHz default, on the other hand, prevents
configuration of the clock rate through assigned-clock-rates, as the
driver immediately overwrites the rate. This behaviour is confusing and
has cost debugging time.

There is little value in a 166 MHz default. All mainline device tree
sources that enable the CSIS specify a clock-frequency explicitly, and
the default wrap clock configuration on supported platforms is at least
as high as 166 MHz. Drop the default, and only set the clock rate
manually when the clock-frequency property is specified.

Link: https://lore.kernel.org/r/20250822002734.23516-10-laurent.pinchart@ideasonboard.com


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>