mtd: rawnand: remove never changed ret variable

The ret variable used for returning value in the function
`meson_nfc_rw_cmd_prepare_and_execute` is never change after initialising.
Therefore, we can remove it safely and return 0 at the end of the function.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210803113300.24230-1-wangborong@cdjrlc.com