mtd: maps: vmu-flash: simplify the return expression of probe_maple_vmu

Simplify the return expression.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200919100854.1639267-1-liushixin2@huawei.com