sisfb: use CONFIG_FB_SIS_301/315 instead of SIS301/315H

There is no need to alias CONFIG #defines.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>