mtd: docg3: fix protection areas reading

The protection areas boundaries were on 16bit registers, not
8bit. This is consistent with block numbers, which can
extend up to 4096 on bigger chips (and is 2048 on the
docg3).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-by: Ivan Djelic <ivan.djelic@parrot.com>
Reviewed-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>