drm/i2c: tda998x: fix bad checksum of the HDMI AVI infoframe

The commit 8c7a075d
"drm/i2c: tda998x: use drm_hdmi_avi_infoframe_from_display_mode()"
also uses hdmi_avi_infoframe_pack() to create the AVI infoframe.
This function sets the checksum of the frame and this breaks
the second calculation of the checksum done in tda998x_write_if().

Fixes: 8c7a075d

 ("drm/i2c: tda998x: use drm_hdmi_avi_infoframe_from_display_mode()")
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>