sparc: kernel/sbus.c: fix memory leakage

The variable iommu and strbuf are not freed properly if it goes to error.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Reviewed-by: Richard Mortimer <richm@oldelvet.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>