sisfb: delete obsolete PCI ROM bug workaround

Delete a workaround for a PCI ROM bug that has been fixed ages ago by
the commit 761a3ac0

.

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