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The cosa driver is rather outdated and does not get built on most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>

drivers/net/wan/Kconfig

@@ -25,7 +25,7 @@ if WAN
 # There is no way to detect a comtrol sv11 - force it modular for now.
 config HOSTESS_SV11
 	tristate "Comtrol Hostess SV-11 support"
-	depends on ISA && m && ISA_DMA_API && INET && HDLC
+	depends on ISA && m && ISA_DMA_API && INET && HDLC && VIRT_TO_BUS
 	help
 	  Driver for Comtrol Hostess SV-11 network card which
 	  operates on low speed synchronous serial links at up to
@@ -37,7 +37,7 @@ config HOSTESS_SV11
 # The COSA/SRP driver has not been tested as non-modular yet.
 config COSA
 	tristate "COSA/SRP sync serial boards support"
-	depends on ISA && m && ISA_DMA_API && HDLC
+	depends on ISA && m && ISA_DMA_API && HDLC && VIRT_TO_BUS
 	---help---
 	  Driver for COSA and SRP synchronous serial boards.
 
@@ -87,7 +87,7 @@ config LANMEDIA
 # There is no way to detect a Sealevel board. Force it modular
 config SEALEVEL_4021
 	tristate "Sealevel Systems 4021 support"
-	depends on ISA && m && ISA_DMA_API && INET && HDLC
+	depends on ISA && m && ISA_DMA_API && INET && HDLC && VIRT_TO_BUS
 	help
 	  This is a driver for the Sealevel Systems ACB 56 serial I/O adapter.