mISDN: Added support for fragmentation of E1 interfaces of hfcmulti driver.

Fragmentation is usefull if multiple devices are connected to an E1
interface. Each fragment will have a subset of the available timeslots.
These devices require a cascde connection or a multiplexer.

Signed-off-by: Andreas Eversberg <jolly@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>