mtd: remove the de-select chip code in nand_release_device()

The nand_get_device() does not select the chip, but nand_release_device()
does de-select the chip. It is really strange.

With the current code, nand_sync() will de-select the chip, even if the chip
has never been selected.

To make the balance of select/de-select chip, it's better to remove the
de-select chip code in nand_release_device() which makes the code more
clear.

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