mtd: rawnand: sunxi: Replace hard coded value by a define

mtd: rawnand: sunxi: Replace hard coded value by a define

The user data length (4) used all over the code hard coded.
And sometimes, it's not that trivial to know that it's the user data
length and not something else.
Moreover, for the H6/H616 this value is no more fixed by hardware, but
could be modified.

Using a define here makes the code more readable.

Suggested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Acked-by: Andre Przywara <andre.przywara@arm.com>