fer fill level to allow
application to control receive latency when using stream oriented serial
protocols that pass receive data to application only after a DMA buffer
fills.  This was previously a compile time option, but run time control is
needed when application changes data rate (and latency requirements) or
uses different data rates on different ports.

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>
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