synclink_gt: improve and simplify write method

Improve write method by allowing multiple HDLC frames to be loaded into tx
DMA buffer ring for continuous frame transmission.  This simplifies the
transmit code by using the common procedures for all serial protocols.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>