[media] staging: st-cec: Use cec_get_drvdata()

Use helper function to get driver private data from CEC
adapter.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>