mtd: rawnand: remove redundant assignment to variable ret

Variable ret is being initialized with a value that is never read
and ret is being re-assigned a little later on. The assignment is
redundant and hence can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>