sisfb: POST should fail if R/W test fails

Currently there is no indication if R/W test fails during POST. This can
happen e.g. when user plugs in a card with unsupported type of memory.
Since the driver will be unusable, it's better to fail the whole POST
if the memory cannot be configured properly.

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