mtd: devices: docg3: Remove unnecessary (void*) conversions

Pointer variables of (void*) type do not require type cast.

Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230529015011.38811-1-suhui@nfschina.com