drivers/video/fbdev/sis: Add missing else

The surrounding code and the fact that the previous two if's are
rendered useless without this "else" (since SIS_340==55 and XGI_20==75
are greater than SIS_661==15) suggests that the "else" is indeed
missing.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>