fs: fat_fs: Fix fs_statvfs when using variable sector sizes

The fs_statvfs function assumes FatFs is configured for a fixed sector
size and therefore may return wrong sector sizes when it is configured
for variable sector sizes instead. Fix that by returning the ssize
variable given in the file system object structure.

Signed-off-by: Markus Fuchs <markus.fuchs@ch.sauter-bc.com>