ieee1394: sbp2: more correct Kconfig dependencies

Make the option SBP2_PHYS_DMA available on all architectures where it
compiles.  This includes x86-64 where I runtime-tested it successfully.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/ieee1394/Kconfig

@@ -97,7 +97,7 @@ config IEEE1394_SBP2
 
 config IEEE1394_SBP2_PHYS_DMA
 	bool "Enable replacement for physical DMA in SBP2"
-	depends on IEEE1394 && IEEE1394_SBP2 && EXPERIMENTAL && (X86_32 || PPC_32)
+	depends on IEEE1394_SBP2 && VIRT_TO_BUS && EXPERIMENTAL
 	help
 	  This builds sbp2 for use with non-OHCI host adapters which do not
 	  support physical DMA or for when ohci1394 is run with phys_dma=0.