media: imx258: Fix getting clock frequency

Obtain the clock frequency by reading the clock-frequency property if
there's no clock.

Fixes: 9fda2533 ("media: i2c: imx258: get clock from device properties and enable it via runtime PM")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>