mtd: remove the "chip" parameter in nand_get_device()

There are two reasons to remove the "chip" parameter in nand_get_device():

[1] The nand_release_device() does not have the "chip" parameter.
[2] We can get the nand_chip by the mtd->priv field.

This patch removes the "chip" parameter in nand_get_device().

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>