Fix compile errors on mismatch between generic HDLC and synclink drivers.

Notes:

generic HDLC support for synclink drivers is *optional* so you can't just
use depend on in Kconfig

This solution is deemed the best after 7 months of review and criticism by
many developers including AKPM.  Read the threads on LKML before posting
about this solution.
Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>