ftdi_sio: Avoid losing bytes at tty-ldisc.

This patch was originally developed by Daniel Smertnig.  I
(Ian Abbott) made a few changes.  It has been tested by both
Daniel and I, at least for raw, non-canonical receive data
processing.

Here is Daniel's original description of the patch:

===
During a project in which I was using a FTDI 232BM to
transmit data at relative high speeds (625kBit/s), I
noticed a problem where data was lost even if flow
control was enabled: The FTDI-Driver receives 512 Bytes
of data over USB at a time, which consists of 8 64-Byte
packets. Subtracting the 2 bytes of status information
included in each packet this gives 496 "real" data
bytes per read.

This data is passed (indirectly, via the flip buffers)
to the tty line discipline which takes care of
throttling when there the free buffer space reaches
TTY_THRESHOLD_THROTTLE (128). Because the FTDI driver
processes up to 496 bytes at a time, throttling won't
happen in time and the line discipline will discard the
remaining bytes.

To avoid this the patch passes data in 62-byte blocks
to the tty layer and checks the available space in the
ldisc-buffers. If there isn't enough free space,
processing the rest of the data is delayed using a
workqueue.

Note: The original problem should be easily
reproducible with a userspace program which does slow &
small reads.
===

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Daniel Smertnig <daniel.smertnig@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>