[SPARC]: change if() BUG(); to BUG_ON in iommu.c

this patch changes if() BUG(); constructs
in iommu.c to BUG_ON(); so it gets save
to define BUG() and BUG_ON() to nullstatements.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>