mtd: rawnand: marvell: prevent harmless warnings

Since the addition of WARN_ON() in nand_subop_get_data/addr_len()
helpers, this driver will produce harmless warnings (mostly at probe)
just because it always calls the nand_subop_get_data_len() helper in
the parsing function (even on non-data instructions, where this value
is meaningless and unneeded).

Fix these warnings by deriving the length only when it is relevant.

Fixes: 760c435e ("mtd: rawnand: make subop helpers return unsigned values")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>