drivers/isdn/hardware/eicon/: remove unused header files

As pointed out by Robert P.  J.  Day, here is a patch to remove unused
header files from Eicon/Dialogic ISDN driver.

Signed-off-by: Armin Schindler <armin@melware.de>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>