drm/i2c: tda998x: fix compiler warning for ssize_t

Stephen Rothwell reports that he sees a compiler warning on x86_64:

drivers/gpu/drm/i2c/tda998x_drv.c: In function 'tda998x_write_avi':
drivers/gpu/drm/i2c/tda998x_drv.c:647:3: warning: format '%d' expects argument of type 'int', but argument 3 has type 'ssize_t' [-Wformat=]
   dev_err(&priv->hdmi->dev, "hdmi_avi_infoframe_pack() failed: %d\n", len);
   ^
Fix this by using the appropriate length modifier.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>