advansys: depends on VIRT_TO_BUS

Fix powerpc allmodconfig build: advansys requires virt_to_bus() but powerpc
doesn't implement it.

Cc: James Bottomley <James.Bottomley@steeleye.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Matthew Wilcox <willy@linux.intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>