mtd: rawnand: mtk: fix possible object reference leak

of_find_device_by_node() takes a reference to the struct device
when it finds a match via get_device, there is no need to call
get_device() twice.
We also should make sure to drop the reference to the device
taken by of_find_device_by_node() on driver unbind.

Fixes: 1d6b1e46 ("mtd: mediatek: driver for MTK Smart Device")
Signed-off-by: Wen Yang <yellowriver2010@hotmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>