net/appletalk: LTPC needs virt_to_bus

The ltpc driver is rather outdated and does not get built on most
platforms because it requires the ISA_DMA_API symbol. However
there are some ARM platforms that have ISA_DMA_API but no virt_to_bus,
and they get this build error when enabling the ltpc driver.

drivers/net/appletalk/ltpc.c: In function 'handlefc':
drivers/net/appletalk/ltpc.c:380:2: error: implicit declaration of function 'virt_to_bus' [-Werror=implicit-function-declaration]
  set_dma_addr(dma,virt_to_bus(ltdmacbuf));
  ^

This adds another dependency in Kconfig to avoid that configuration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>