mtd: nand: remove hard-coded NAND ids length

This commit removes hard-coded '8' used for looping into
struct nand_chip.id.data array.

NAND_MAX_ID_LEN has been introduced by Artem Bityutskiy in
53552d22 for defining ids length in nand_flash_ids[] list.

This commit unifies ids length in nand base driver.

Signed-off-by: Jean-Louis Thekekara <jeanlouis.thekekara@parrot.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>