powerpc/pseries/vas: Call misc_deregister if sysfs init fails

Undo effects of misc_register if sysfs init fails after
misc_register.

Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220511033507.2745992-1-zhengbin13@huawei.com