ssb: remove unreachable code

The return value of ssb_bus_unregister can only be 0 or -1, so this
condition if (err == -EBUSY) will not hold, so delete it.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Michael Büsch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1621306352-3632-1-git-send-email-tiantao6@hisilicon.com