fbdev: sisfb: use explicitly signed char

With char becoming unsigned by default, and with `char` alone being
ambiguous and based on architecture, signed chars need to be marked
explicitly as such. This fixes warnings like:

drivers/video/fbdev/sis/init301.c:3549 SiS_GetCRT2Data301() warn: 'SiS_Pr->SiS_EModeIDTable[ModeIdIndex]->ROMMODEIDX661' is unsigned

Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-usb@vger.kernel.org
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Helge Deller <deller@gmx.de>

Author: zx2c4

drivers/usb/misc/sisusbvga/sisusb_struct.h

@@ -91,7 +91,7 @@ struct SiS_Ext {
 	unsigned char VB_ExtTVYFilterIndex;
 	unsigned char VB_ExtTVYFilterIndexROM661;
 	unsigned char REFindex;
-	char ROMMODEIDX661;
+	signed char ROMMODEIDX661;
 };
 
 struct SiS_Ext2 {

drivers/video/fbdev/sis/vstruct.h

@@ -148,7 +148,7 @@ struct SiS_Ext {
 	unsigned char  VB_ExtTVYFilterIndex;
 	unsigned char  VB_ExtTVYFilterIndexROM661;
 	unsigned char  REFindex;
-	char           ROMMODEIDX661;
+	signed char    ROMMODEIDX661;
 };
 
 struct SiS_Ext2 {