drm: bridge: dw-hdmi: Constify dw_hdmi_i2s_ops

The only usage of dw_hdmi_i2s_ops is to assign its address to the ops
field in the hdmi_codec_pdata struct, which is a const pointer. Make it
const to allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201004200653.14702-1-rikard.falkeborn@gmail.com