isdn: Make CONFIG_ISDN depend on CONFIG_NETDEVICES

It doesn't make much sense to enable ISDN services if you don't
intend to connect to a network. Therefore insisting that ISDN
depends on NETDEVICES seems logical. We can then remove any
guards mentioning NETDEVICES inside all subordinate drivers.

This also has the nice side-effect of fixing the warning below
when ISDN_I4L && !CONFIG_NETDEVICES at compile time.

This patch fixes:
drivers/isdn/i4l/isdn_common.c: In function ‘isdn_ioctl’:
drivers/isdn/i4l/isdn_common.c:1278:8: warning: unused variable ‘s’ [-Wunused-variable]

Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>