mtd: jz4780_nand: remove useless mtd->priv = chip assignment

As of commit 2d3b77ba ("mtd: nand: update mtd_to_nand()"), this
assignment isn't necessary, since struct mtd_info is embedded in struct
nand_chip.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: Alex Smith <alex@alex-smith.me.uk>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>