mtd: rawnand: legacy: Drop useless test in nand_legacy_set_defaults()

nand_legacy_set_defaults() returns directly if chip->exec_op != NULL,
no need to test !chip->exec_op after that.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>