mtd: m25p80: Fix JEDEC ID for AT26DF321

The last byte of the ID should be zero for this chip. Was added in
commit d0e8c47c . Reported by Tomi
Varjo.

Signed-off-by: Aleksandr Koltsoff <aleksandr.koltsoff@ebts.fi>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>