isdn: mISDN HFC PCI support depends on virt_to_bus()

On powerpc (allyesconfig build) we get this error:

drivers/isdn/hardware/mISDN/hfcpci.c:1991: error: implicit declaration of function 'virt_to_bus'
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/isdn/hardware/mISDN/Kconfig

@@ -7,6 +7,7 @@ config MISDN_HFCPCI
 	tristate "Support for HFC PCI cards"
 	depends on MISDN
 	depends on PCI
+	depends on VIRT_TO_BUS
 	help
 	  Enable support for cards with Cologne Chip AG's
           HFC PCI chip.