staging: xgifb: eliminate XGIfb_CRT2_write_enable variable

Replace XGIfb_CRT2_write_enable with IND_XGI_CRT2_WRITE_ENABLE_315.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>