synclink_gt: improve TIOCOUTQ accuracy

Improve the accuracy of TIOCOUTQ value as implemented in chars_in_buffer()
method by walking and counting tx DMA buffers, reading controller tx FIFO
level and accounting for controller tx shift register.  The greatly
improves application control of transmit latency at lower data rates.

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>