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 <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Helge Deller <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Jason A. Donenfeld <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/usb/misc/sisusbvga/sisusb_struct.h

···
 	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

···
 	unsigned char  VB_ExtTVYFilterIndex;
 	unsigned char  VB_ExtTVYFilterIndexROM661;
 	unsigned char  REFindex;
-	char           ROMMODEIDX661;
+	signed char    ROMMODEIDX661;
 };
 
 struct SiS_Ext2 {