[POWERPC] Fix viodasd geometry calculations

Commit a885c8c4

 that introduced the
getgeo block device method changed the fallback number of sectors and
introduced a bug into the fallback cylinder number calculation.

Thanks to Rusty Russell for noticing this.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>