[ISDN] include/linux/isdn.h: remove dead code

This patch remove the usage of a nonexisting kconfig variable.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed