net: wan: add missing virt_to_bus dependencies

The cosa driver is rather outdated and does not get built on most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/wan/cosa.c: In function 'tx_interrupt':
drivers/net/wan/cosa.c:1768:3: error: implicit declaration of function 'virt_to_bus'
   unsigned long addr = virt_to_bus(cosa->txbuf);
   ^

The same problem exists for the Hostess SV-11 and Sealevel Systems 4021
drivers.

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>