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>