ISDN: Make isdnhdlc usable for other ISDN drivers

isdnhdlc is useful for other ISDN drivers as well.
Move the include file to a central location and the source
to the central isdn location.

Signed-off-by: Karsten Keil <keil@b1-systems.de>
diff --git a/drivers/isdn/i4l/Makefile b/drivers/isdn/i4l/Makefile
index 49a06c0..cb9d3bb 100644
--- a/drivers/isdn/i4l/Makefile
+++ b/drivers/isdn/i4l/Makefile
@@ -4,6 +4,7 @@
 
 obj-$(CONFIG_ISDN_I4L)		+= isdn.o
 obj-$(CONFIG_ISDN_PPP_BSDCOMP)	+= isdn_bsdcomp.o
+obj-$(CONFIG_ISDN_HDLC)		+= isdnhdlc.o
 
 # Multipart objects.