Add custom HDLC idle pattern feature.

It allows the user to specify an arbitrary 8 or 16 bit repeating pattern on
the transmit data pin between HDLC frames.

In most cases the idle pattern is continuous ones or flags as supported by off
the shelf synchronous controllers and defined in the ISO3309 standard.  Some
applications (radio/satellite modems, connections to legacy military hardware)
require non-standard patterns.
Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>