md: changed the switch of RAID_VERSION to if

There is only one case of this 'switch'. Change it to 'if'.

Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240226031444.3606764-3-linan666@huaweicloud.com