isdn: hisax: netjet requires VIRT_TO_BUS

Disabling CONFIG_VIRT_TO_BUS on ARM showed that the hisax netjet
driver depends on this deprecated functionality but is not
marked so in Kconfig.

Rather than adding ARM to the already long list of architectures
that this driver is broken on, this patch adds 'depends on
VIRT_TO_BUS' and removes the dependency on !SPARC, which is
also implied by that.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>