mtd: doc2000: fix uninitialized variable in doc_ecc_decode()

The variable 'syn' was being used uninitialized.  Also
fixed incorrect use of syn[] vs s[].

Tested on powerpc board with 64MiB DOC2000.

Signed-off-by: Mark Ware <mware@elphinstone.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>