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/hisax/Kconfig b/drivers/isdn/hisax/Kconfig
index 7832d8b..3464ebc 100644
--- a/drivers/isdn/hisax/Kconfig
+++ b/drivers/isdn/hisax/Kconfig
@@ -391,6 +391,7 @@
 config HISAX_ST5481
 	tristate "ST5481 USB ISDN modem (EXPERIMENTAL)"
 	depends on USB && EXPERIMENTAL
+	select ISDN_HDLC
 	select CRC_CCITT
 	select BITREVERSE
 	help
@@ -418,11 +419,6 @@
 	  (the latter also needs you to select "ISA Plug and Play support"
 	  from the menu "Plug and Play configuration")
 
-config HISAX_HDLC
-	bool
-	depends on HISAX_ST5481
-	default y
-
 config HISAX_AVM_A1_PCMCIA
 	bool
 	depends on HISAX_AVM_A1_CS