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>