Staging: xgifb: vb_setmode.c: Fix checkpath warning

Fix the following checkpatch.pl warning:

Avoid multiple line dereference - prefer
'XGI330_RefIndex[RefreshRateTableIndex+(*i)].Ext_InfoFlag'

Signed-off-by: Manoj Sawai <mas@iitkgp.ac.in>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>