mtd: nand: Fix build issues due to an anonymous union

GCC-4.4.4 raises errors when assigning a parameter in an anonymous
union, leading to this kind of failure:

drivers/mtd/nand/marvell_nand.c:1936:
    warning: missing braces around initializer
    warning: (near initialization for '(anonymous)[1].<anonymous>')
    error: unknown field 'data' specified in initializer
    error: unknown field 'addr' specified in initializer

Work around the situation by naming these unions.

Fixes: 8878b126 ("mtd: nand: add ->exec_op() implementation")
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Tested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>