media: camss: csiphy: Ensure clock mux config is done before the rest

Add a write memory barier after clock mux config and before the rest
of the csiphy config.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>