sbawe: fix memory detection part 2

The patch "sbawe: fix memory detection" fixed detection
for memoryless SB32 cards but broke detection of memory
above 512KB. This patch fixes the regression.

The patch has been tested on the SB32 card (CT3670) with
0MB, 2MB and 8MB memory installed.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>