mtd: onenand: samsung: remove set but not used variable

Fixes gcc '-Wunused-but-set-variable' warning:
drivers/mtd/nand/onenand/samsung_mtd.c: In function s3c_onenand_check_lock_status:
drivers/mtd/nand/onenand/samsung_mtd.c:731:6: warning: variable tmp set but not used [-Wunused-but-set-variable]

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>