All drivers that are relevant for rpc or footbridge have stopped
using virt_to_bus a while ago, so we can remove it and avoid some
harmless randconfig warnings for drivers that we do not care about:

drivers/atm/zatm.c: In function 'poll_rx':
drivers/atm/zatm.c:401:18: warning: 'bus_to_virt' is deprecated [-Wdeprecated-declarations]
   skb = ((struct rx_buffer_head *) bus_to_virt(here[2]))->skb;

FWIW, the remaining drivers using this are:

ATM:  firestream, zatm, ambassador, horizon
ISDN: hisax/netjet
V4L:  STA2X11, zoran
Net:  Appletalk LTPC, Tulip DE4x5, Toshiba IrDA
WAN:  comtrol sv11, cosa, lanmedia, sealevel
SCSI: DPT_I2O, buslogic
VME:  CA91C142

My best guess is that all of the above are so hopelessly obsolete that
we are best off removing all of them form the kernel, but that can be
done another time.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>