dmascc: add CONFIG_VIRT_TO_BUS dependency

Many architectures don't define virt_to_bus() any more, as drivers
should be using the dma-mapping interfaces where possible:

In file included from drivers/net/hamradio/dmascc.c:27:
drivers/net/hamradio/dmascc.c: In function 'tx_on':
drivers/net/hamradio/dmascc.c:976:30: error: implicit declaration of function 'virt_to_bus'; did you mean 'virt_to_fix'? [-Werror=implicit-function-declaration]
  976 |                              virt_to_bus(priv->tx_buf[priv->tx_tail]) + n);
      |                              ^~~~~~~~~~~
arch/arm/include/asm/dma.h:109:52: note: in definition of macro 'set_dma_addr'
  109 |         __set_dma_addr(chan, (void *)__bus_to_virt(addr))
      |                                                    ^~~~

Add the Kconfig dependency to prevent this from being built on
architectures without virt_to_bus().

Fixes: bc1abb9e ("dmascc: use proper 'virt_to_bus()' rather than casting to 'int'")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hamradio/Kconfig

@@ -48,6 +48,7 @@ config BPQETHER
 config DMASCC
 	tristate "High-speed (DMA) SCC driver for AX.25"
 	depends on ISA && AX25 && BROKEN_ON_SMP && ISA_DMA_API
+	depends on VIRT_TO_BUS
 	help
 	  This is a driver for high-speed SCC boards, i.e. those supporting
 	  DMA on one port. You usually use those boards to connect your