staging: xgifb: use proper typing for TV data

There's a dangerous cast from XGI330_TVDataStruct to SiS_TVData. Use
SiS_TVData everywhere, also the data can be made const.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>