mtd: docg3: fix tracing of IO in writeb

Writeb was incorrectly traced as a 16 bits write, instead of
a 8 bits write. Fix it by tracing the correct width.

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>